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Study on status characteristics and oxidation reactivity of biodiesel particulate matter
Fuel 218 (2018) 218–226
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Full Length Article
Study on status characteristics and oxidation reactivity of biodiesel particulate matter
T
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Ruina Li , Zhong Wang School of Automobile and Traffic Engineer, Jiangsu University, Zhenjiang 212013, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Diesel engine Biodiesel PM Status characteristics Oxidation reactivity
Biodiesel is a kind of clean and renewable fuel for diesel engine. The source of biodiesel affects the component and physicochemical properties of biodiesel, and it also influences the formation of particulate matter (PM) during the combustion process. In this paper, the PM of diesel and 5 kinds of biodiesel from different sources were collected. In addition, the effects of biodiesel from different sources on morphology and PM diameter were analyzed. Taking the biodiesel made from waste cooking oil as the research object, the PM of diesel engine fueled with diesel/biodiesel blend ratio of 0%, 20%, 50%, 100% were collected by the test bench at engine speed 2000 r/min and engine torque 12.27 N·m. The effects of biodiesel on morphology, diameter, elements, functional groups and component of PM were analyzed. Results show that the average PM diameter of biodiesel from different sources is between 20 and 90 nm. The PM diameter of biodiesel made from cottonseed is larger than that from other different sources. From the test, the average PM diameter of diesel is 26.27 nm. With the increase of biodiesel blend mixing ratio from 0% to 20%, 50% to 100%, the diameter of PM is decreased by 7.9%, 17.2% and 24.2% compared with that of diesel respectively and the microscopic morphology of PM changes from ring to cluster. The analysis of X ray energy spectrometer shows that the content of C in PM is reduced. However, the content of O, Na, Cu, Al, Ba and Zn is increased. The analysis of X-ray absorption near edge structure shows that the content of “graphite” C ] C, phenol CeOH, ketone C ] O, aliphatic C]C in PM is decreased gradually, and the content of aliphatic hydrocarbon CeH and carboxyl C]O is increased. With the increase of biodiesel blend mixing ratio, the content of volatile organic compounds (VOF) is increased and the content of soot is decreased. When biodiesel mixing ratio reaches 50%, Ti, Tm, Tb and E of PM are decreased obviously from thermo-gravimetric analyzer test, it means that oxidation of PM was more actively.
1. Introduction
Compared with diesel, biodiesel can effectively reduce the emission of unburned hydrocarbons (HC), carbon monoxide (CO) and PM in diesel engine [2–6]. PM is mainly composed of VOF, soot and metal components. The component and structure of PM have a great influence on the oxidation activity of PM, and the oxidation activity of PM influences on the regeneration of diesel particulate filter (DPF) carrier. For the PM with higher oxidation activity, the regeneration temperature is lower and the regeneration efficiency is higher [7]. Biodiesel made from different raw materials contains different types of esters, and the physicochemical properties will be different from each other, leading to different combustion parameters of biodiesel and different oxidation activity of PM. Abdalla [8] studied the properties for diesel, biodiesel and blended fuels. The results indicated that a linear increase with increasing the biodiesel percentage in the blend. Sulphur content decreased continuously with increasing the biodiesel percentages in the blend while the acid content increased with increasing biodiesel ratio. Madheshiya [9] studied the soot emissions of diesel
In 2016, China's dependence on foreign oil has reached 65.4% [1]. Finding clean alternative fuels for diesel engines has become an important topic among scholars. Biodiesel is prepared by transesterification of vegetable oils or animal fats. The raw materials of biodiesel don’t contain sulfur, aromatic compounds and other harmful substances. Diesel engine fueled with biodiesel fuel could reduce particulate matter (PM) emission, and it can relieve haze and air problems. Besides, the degradation rate of biodiesel is as high as 98%, which is 2 times of that of mineral diesel. It is a kind of high quality green environmental protection renewable energy [4–7]. In China, waste cooking oil is the main source for biodiesel. At present, Chinese biodiesel has achieved large-scale production and increased year by year. In 2016, China had more than 50 biodiesel product companies with a total capacity of more than 3.5 million tons. Biodiesel can be mixed with diesel or used directly in diesel engines. ⁎
Corresponding author. E-mail address: [email protected] (R. Li).
https://doi.org/10.1016/j.fuel.2018.01.041 Received 7 November 2017; Received in revised form 2 January 2018; Accepted 11 January 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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Fe, Cr, Cu, Mn, Pb, Ni, Ba and Cd) and inorganic anions (such as nitrates, chlorides, fluorides and sulphates) were compared for biodiesel/ diesel blend and diesel. The results showed that biodiesel origin particulate contained slightly lower amount of trace metals and inorganic ions compared with diesel for similar operating conditions except nitrates. According to the analysis above, biodiesel has a wide range of sources and complex components, and it will influence the formation of PM from diesel engine. The oxidation activity of PM is closely related to the microstructure, such as elements, component, and functional group. However, the PM oxidation activity has most important effect on exhaust emission of the engine, and it affects after-treatment of diesel engine. There are no enough reports to explanation the status characteristics and oxidation activity of biodiesel PM. From this view points of biodiesel sources and fuel characteristics, it is necessary to carry out the research on the status characteristics and oxidation reactivity of biodiesel PM. Friendly environment and stricter engine emission stander requires lower diesel engine particulate emissions. DPF is one of the effective way to reduce the PM. With the time going on, the PM adsorbed in the DPF will cause problems such as blockage of the DPF, etc. Performance of diesel engine will be reduced. This involves the problem of DPF regenerated. The objective of this research is to study the influence of biodiesel on the status characteristics and oxidation reactivity of PM. In this paper, the PM diameter distribution of diesel oil and biodiesel made from five different sources were investigated by the combustion test at normal temperature and pressure. Then the waste cooking oil biodiesel was selected to add into diesel, and PM of diesel/biodiesel blend with different blending ratio were collected on engine bench test. By means of high resolution transmission electron microscopy (TEM), X ray energy spectrometer, thermogravimetric analyzer and X-ray absorption near edge structure (XANES) test, the micro morphology, element type, chemical component and functional group of the PM were measured, and the effect of biodiesel on the formation of PM characteristics was discussed.
engine fueled with diesel, waste cooking oils and mustered oil. The results showed that soot emissions were alike for diesel, waste cooking oils, and mustered oils at low load, but at higher load diesel had an exponential increment in soot emissions. Zhao [10] studied analysis of fuel structures on engine soot particles’ mass and size using diesel and different levels of unsaturated biodiesel fuels through the numerical simulation. The results showed that the biodiesel fuel with a higher fraction of unsaturated fatty acid methyl esters (more double carbon bonds C]C) contributed more to the formation of soot precursors, thus producing a higher amount of soot particles in mass and numbers, accelerating soot particle nucleation and soot surface growth. Giakoumis [11]studied the effects of diesel/biodiesel blends on the exhaust emissions of diesel engines operating under transient conditions. The results showed that the use of biodiesel blends, regardless of their source material, resulted in dramatic PM mass reductions ranging from 67% to 74%. These reductions could be attributed to the increased oxygen concentration in the biodiesel blend, which reduced locally fuel-rich regions and limits soot nucleation in the formation process. Grigoratos [12]tested diesel as well as three blends of rapeseed methyl ester at 10%, 30% and 50% v/v with three Euro 4+ compliant vehicles. The result showed higher oxidative activity with increasing biodiesel blending when testing two light-duty vehicles with and without a DPF over the NEDC and Artemis cycles. Agudelo etc. [13] compared the oxidation activity of PM produced by diesel, palm oil biodiesel and jatropha oil biodiesel. The results showed that PM oxidation activity of Jatropha curcas oil biodiesel was the highest, followed by palm oil biodiesel, and diesel. Karavalakis [14] assessed PM on two heavy-duty trucks fueled with ultra-low sulfur diesel and biodiesel blends as well as the oxidative potential of the PM measured by means of the dithiothreitol assay. The results showed a reduction on oxidative potential of PM with the use of biodiesel blends relative to ultra-low sulfur diesel. The oxidation activity of PM is closely related to the microstructure of PM. Disordered carbon atoms, which are located in PM microcrystalline graphite layer, have unpaired sp2 electrons, are more easily to form a covalent bond with the chemical adsorption oxygen. However, the ordered carbon atoms in microcrystalline graphite layer only with sharedπelectron, are difficult to combine with oxygen. The surface functional group on the edge of the PM carbon layer is more likely to react with the oxidized gas to form CO or CO2, and to make the carbon atoms fall off the original chemical bond. Cain etc. [15]studied the distribution of functional groups on soot PM. The results showed that many types of functional groups were present on the surface of the soot PM, including aliphatic CeH functional group, aromatic CeH functional group and oxygen-containing group (C]O, CeOOH, CeOH, CeOeC). Among these functional groups, the relative content of aliphatic CeH functional group was the highest. Nabi [6]studied PM and particulate number (PN) of biodiesel/diesel blends in a six-cylinder turbocharged diesel engine with a high-pressure common rail injection system in compliance with a 13-Mode European Stationary Cycle. Results showed that with the addition of biodiesel, a significant reduction in both PM and PN emissions was observed relative to those of diesel. A maximum reduction of 84% PM and 88% PN was observed by using the biodiesel blends. Wei [16] studied the influence of waste cooking oil biodiesel on particulate emissions of a direct-injection diesel engine. The results showed that the addition of biodiesel reduced the weighted particle mass concentration and the weighted geometric mean diameter of the particles. The influence of biodiesel on the investigated emissions was proportional to the biodiesel content in the tested fuels. Gali [17] studied the chemical components of solid and semi-volatile PM from diesel and biodiesel blend. The results showed that PM emission was lower for biodiesel/diesel blend (30% biodiesel added) in general. PM for biodiesel/diesel blend contained 19% higher organic compounds in semi-volatile PM compared with diesel, which on the other hand noted 24% more solid PM. Shukla [18] studied the trace metals and ions in particulates of engine fueled with Karanja biodiesel blend (20% v/v biodiesel in the blend fuel), and 9 commonly present trace metals (Ca,
2. Analysis of biodiesel quality and PM formation factors 6 kinds of fuel were used in this test, including 0# diesel, waste cooking oil biodiesel, rapeseed oil biodiesel, soybean oil biodiesel, cottonseed oil biodiesel and palm oil biodiesel. 0# diesel is commercial diesel, soybean oil biodiesel is produced by Hainan positive and Bio Energy Limited, waste cooking oil biodiesel is produced by Jiangsu Carter Petroleum New Energy Co., Ltd. The remaining 3 kinds of biodiesel are prepared in Jiangsu biodiesel power machinery application engineering center. The physicochemical parameters of the fuels are listed in Table 1. The formation mechanism of PM is complex, which is greatly affected by the combustion process. It is generally believed that soot size and amount of formation are related to temperature, pressure, time, oxygen content, fuel properties and so on. Local high temperature, hypoxia, cracking and dehydrogenation are benefit for the generation of PM. The carbon structure of PM has an important effect on the oxidation of PM. The PM whose carbon atoms are arranged orderly, like in a graphite structure, are difficult to be oxidized in the combustion process [19]. Thus, the properties of the fuel which will influence the formation of PM are analyzed below. 2.1. Oxygen content The area in front of the burning flame is rich of oxygen, and soot is mainly generated on the side of the flame front. The average oxygen content in biodiesel is around 11%. The oxygen in biodiesel can improve the situation of oxygen deficiency and promote combustion. So, combustion condition of biodiesel is better compared with diesel and less soot is formed. Bedjanian’s study showed that fuel quality played an important role in the formation of PM. Oxygen in the fuel could 219
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Table 1 Physicochemical properties of the six fuels. Parameters
0# diesel
Waste cooking oil biodiesel
Rapeseed oil biodiesel
Soybean oil biodiesel
Cottonseed oil biodiesel
Palm oil biodiesel
ρ20/(kg/m3) V40/(mm2/s) Cetane number w(S)/% w(C)/% w(H)/% w(O)/% Lower heat value /(kJ/kg)
830 2.4 45 0.2 87.0 12.6 0.4 42.5
877.5 4.1 52 0.0007 77.1 12.2 10.1 37.99
876.4 4.0 54.4 0.001 68.9 10.6 10.9 38.61
881 4.47 51.2 0.001 67.89 7.77 13.5 37.27
880 3.923 58 0.007 78.1 11.5 10 38.13
874 4.38 58.3 0.004 75.6 13 12.8 37.1
resulted in a smaller amount of PM generated by biodiesel. Labeckas carried out a diesel engine test with ethanol and biodiesel, and studied the effects of the cetane number on exhaust smoke. The results showed that smoke can be decreased by 1.7 times or by 34.9%, when running the engine with the higher cetane number (67.3) blend, compared with a lower cetane number blend [25]. Vallinayagam’s study showed that fuel with higher cetane number leaded to less soot [26].
significantly inhibit the nucleation of PM, and the increase in oxygen content could reduce soot volume fraction and PM size [20]. Ramin tested the PM oxidation characteristics of diesel engine by using a thermogravimetric analyzer. The results showed that the microstructure of PM and the oxygen-containing groups had great influence on the oxidation activity of PM. The more regular the PM structure was, the less was the number of oxygen groups and the lower was the activity of oxidation [21]. Wall launched a study on the microstructure and oxidation characteristics of PM carried out on a diesel engine fueled with oxygen added diesel fuel. The results showed that the addition of oxygen into the fuel leaded to an larger PM layer distance and curvature, but a smaller crystallite size [22].
3. Experimental scheme and equipment 3.1. Experimentation methodology (1) In order to study the effects of biodiesel sources on the PM formation, the PM samples of diesel, waste cooking oil biodiesel, rapeseed oil biodiesel, soybean oil biodiesel, cottonseed oil biodiesel, palm oil biodiesel were collected in an atmospheric combustion test. The micro morphology and PM diameter were analyzed through TEM. (2) As biodiesel in China is mostly made from waste cooking oil biodiesel, in order to investigate the effects of biodiesel on the microstructure and component characteristics of PM, a diesel engine bench test was carried out to collect PM samples of diesel/ waste cooking oil biodiesel blend. The experiment was carried out in the 186F single cylinder direct injection diesel engine. A scheme of the test engine set up is shown in Fig. 1 and the specifications of the test engine are provided in Table 2. The experiment was carried on a stable diesel engine, running under the condition of 2000 r/min and 12.27 N·m. The biodiesel mixing ratios of experimental fuel were 0%, 20%%, 50% and 100%, recorded as B0, B20, B50 and B100, respectively. The micro-orifice uniform deposition impactor (MOUDI) was used to collect PM of the blends. During the sampling, the standard aluminum foil filter paper (47 nm × 8, MSP) was used to collect PM samples. The sampling flow was 30 L/min, and the sampling time was 20 min. In order to reduce the
2.2. Aromatic hydrocarbon content Aromatic hydrocarbon, which includes single ring, double rings as well as a small number of tricyclic structure, is difficult to evaporate, mix and combust. At higher temperature, the ring structure of aromatic hydrocarbon does not prone to cleavage, but it occurs condensation directly and generate soot precursors of polycyclic aromatic hydrocarbons. Therefore, the more aromatic hydrocarbon is contented, the greater is the amount of generated soot. The mass fraction of aromatic hydrocarbons in diesel oil is about 5.82%-21.39%. The main component of biodiesel is fatty acid, which contains almost no aromatic hydrocarbon. Therefore, the amount of soot, generated by biodiesel is smaller than that of diesel fuel. Xie integrated a skeletal polycyclic aromatic hydrocarbon model with a toluene reference fuel oxidation model. The results show that the predicted benzene concentration in the oxidation of alkanes and aromatic hydrocarbons indicates that the molecular structure of the fuel significantly affects the PAH formation pathway [23]. 2.3. Sulfur content Sulfur content which is an important indicator of oil products, is directly related to the PM amount of diesel engine. The increase of sulfur content of diesel results in an increase of sulfate in the combustion process. The higher sulfur concentration on the soot surface increases the PM size. In addition, the intermediate product of sulfur reactions can promote soot formation and increase the total number of PM. In China, the national standard V for vehicle diesel sulfur content limit is 10 mg/kg. As biodiesel is made from vegetable oil and animal fats, there is no sulfur in biodiesel. So, the PM size of biodiesel is smaller than that of diesel and the amount of PM is less than that of diesel. 2.4. Cetane number
1- Eddy current dynamometer, 2- Diesel engine, 3- Optical encoder, 4-Pressure sensor, 5-Smoke intensity, 6- Exhaust analyzer, 7- Cooler, 8- Float flowmeter, 9- Interial impactor, 10- Up pressure gauge, 11- Down pressure gauge, 12- Flow valve, 13- Pump
Cetane number is a measure of the ignition performance of diesel fuel. The cetane number of the fuel is high, the ignitability is better. Cetane number also affects the emissions of the diesel engine. The research showed that increasing the cetane number of diesel fuel could reduce CO, HC and soot [24]. The Cetane number of 0# diesel is about 45, the cetane number of biodiesel is between 51 and 59, which
Fig. 1. Schematic of test engine set up.
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before (I0) and after (I1) sample was obtained. Then the light absorption coefficient could be obtained according to the formula μ(E) = ln(I1/I0). The PM functional groups of B0, B20, B50 and B100 were measured.
Table 2 Engine specifications of 186F. Type
Direct-injected, 4 stokes, air-cooled, natural aspiration
Number of cylinders Cylinder bore (mm) × stroke (mm) Compression ratio Rated power (kW)/speed(r/min) Maximum power(kW) /speed (r/min) Nozzle number × orifice diameter (mm) Injection start (°CA BTDC)
1 86 × 70 19 5.7/3000 6.9/1800 4 × 0.24 12
3.2.4. Thermogravimetric analyzer The thermogravimetric analysis was used to test the quality of the sample in the heating furnace with time and temperature. The experiment was carried out on TGA/DSC1 thermogravimetric analysis produced by Mettler-Toledo in Switzerland. Temperature range: room temperature to 1600 °C, Temperature accuracy of ± 0.3 °C, heating rate 0.1–150 °C/min, Sample load 0–1000 mg, Balance sensitivity 0.1 μg, temperature resolution 0.0001 °C. The particle sample is weighed by the MX5 type micro-electronic balance of Mettler-Toledo company, Switzerland. Weighing range 0–5.1 g, minimum score 1 μg. The combustion temperature ranges of volatile matter and soot are different for diesel engine combustion particles. Therefore, through the thermogravimetric test, volatile substances and soot can be distinguished. During the experiment, the oxidation gas was O2 and the protective gas N2. The oxidation gas and the protective gas flow rate was 50 L/min, the heating rate was selected at 20 K/min, the temperature interval was 20–750 K and the initial mass of the PM was about 2 mg.
measurement error, multiple measurements were carried out at the same working condition to calculate the average values. Meanwhile, the PM collected at different working conditions were prepared for TEM, X ray spectrum, thermalgravimetricr and XANES analysis. 1- Eddy current dynamometer, 2- Diesel engine, 3- Optical encoder, 4- Pressure sensor, 5- Smoke intensity, 6- Exhaust analyzer, 7- Cooler, 8- Float flowmeter, 9- Interial impactor, 10- Up pressure gauge, 11Down pressure gauge, 12- Flow valve, 13- Pump
4. Results and discussion
3.2. Experimental equipments
4.1. Effects of biodiesel sources on PM microstructure
3.2.1. TEM The experiment was carried out on JEM-2100 (HR) high resolution transmission electron microscopy. The acceleration voltage was 200 kV, the point resolution 0.24 nm, the lattice resolution 0.14 nm, and the magnification 2000–1,500,000 times. Before the test, 1 mg of the PM was taken and placed in a centrifuge tube. A small amount of methylene chloride was dripped into the centrifuge tube, and then the centrifuge tubes were subjected to ultrasonic treatment for 20 min. After dropping 5 drops of ultrasound solution on the HRTEM copper mesh (230 mesh), bake the copper mesh under a hot lamp. Then the suitable acceleration voltage, condenser current, working distance, objective diaphragm and scanning speed were selected to ensure the image can satisfy the requirement of study.
The formation of PM undergoes nucleation, surface growth and condensation, agglomeration, and oxidation. After the formation of “core”, on the one hand, the gas phase component moves to the core surface, on the other hand, it is the collision and condensation between the core PM, so that the PM grows up and becomes approximately spherical. Fig. 2 shows the PM photograph of biodiesel from six different sources with a magnification of 50,000 times. The particle morphology shows different distribution characteristics of particle morphology. It can be seen that the basic PM are mostly spherical or ellipsoidal, where each of them are containing hundreds of quasi spherical basic PM of unequal size. These basic carbon PM accumulates under the adhesion force of Van der Waals force, electrostatic force, liquid bridging force and so on, to form different PM groups with different tightness [27,28]. In terms of density, the arrangement of PM in biodiesel is relatively close, and the PM in diesel oil is arranged loosely. According to the degree of caking, the PM of biodiesel are stronger, and they are mostly bonded, while the bonding of diesel PM is weak. Among the 5 kinds of biodiesel, the PM produced by waste cooking oil biodiesel has the smallest particle size and the highest agglomeration, and the PM produced by cottonseed oil biodiesel has the biggest particle size and the lowest agglomeration. The higher the unsaturated degree of the fuel molecules, the lower the cetane number, the worse the ignition, and worse ignition will lead to more and bigger PM. Study shows that the waste cooking oil biodiesel contains about 54.8% unsaturation fatty acid methylester, and cottonseed oil biodiesel contains about 60.3% unsaturation fatty acid methylester [29]. This maybe the reason why the PM produced by waste cooking oil biodiesel is smaller and the PM produced by cottonseed oil biodiesel is bigger. According to Fig. 2, a square contained about 20 particles in the TEM image was chosen randomly to measure the diameter of each particle. After getting the diameter for each particle, the average PM diameter could be calculated. For each TEM image, the statistics and calculation were repeated above 10 times. At last, the distribution of particle size and the final average PM diameter could be got. Fig. 3 shows the PM diameter of six kinds of fuel. It can be seen that the diameter of diesel and biodiesel PM is mainly distributed between 10 and 100 nm and most of them are between 20 and 90 nm. The fuel has a great influence on the average PM diameter and PM distribution. The PM diameter of biodiesel made from cottonseed oil is larger than that of diesel by about 4–17 nm. The main reason is that PM of cottonseed oil
3.2.2. X ray energy spectrometer Element species of PM were tested using a S-4800 field emission scanning electron microscope and a X ray energy spectrum analyzer, produced by Japan's Hitachik High-tech company. The electron emission source of the electron microscope or was cold field emission, and the objective lens was semi submerged. Two times electronic resolution was 1.0 nm (15 kV), 2.0 nm (1 kV), backscattered electron resolution was 3.0 nm (15 kV) and accelerating voltage was 0.5–30 kV (0.1 kV/ step, variable). The magnification was 30–800,000. The elemental analysis range of X ray spectrometer is Be4-U92. Before the test, a small amount of PM was placed into acetone solution and was treated with ultrasonic waves. After treatment the sample was placed on silicon wafers to dry, the samples were sprayed with metal by an ion sputtering apparatus. 3.2.3. XANES The XANES experiment of diesel engine PM was carried out in the BL08U1-A of Shanghai Synchrotron. The electron energy of storage ring was 2.5 Gev, flux was 150–300 mA, photon energy range 250–2000 eV, photon flux 106–109 (photons/s), the pressure of sample room is about 10–6 Torr, natural emissivity is 3.9 nm·rad and the energy resolution was greater than 1000. The sample frame can move in three dimensions and can be rotated around the vertical axis. The standard reference materials are oxalic acid, sucrose and graphite. During the experiment, the near edge X ray absorption spectrum of C on K edge was scanned and recorded by the step length of 0.2 eV, and the X ray intensity of 221
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Fig. 2. Photographs of six kinds of particulate (×50,000).
Diesel
Waste cooking oil biodiesel
Soybean oil biodiesel
Rapeseed oil biodiesel
Cottonseed oil biodiesel
Palm oil biodiesel
diesel, biodiesel has a greater viscosity. This will lead to a poorer droplet breakup, diffusion and atomization. The increase of unburned fuel and VOF will increase the number of nucleated PM, as well as the collision frequency and coagulation rate between PM, resulting in the microscopic morphology of PM develops to cluster. The PM aggregates show different tightness at the junction, where the deeper areas indicate a overlapping of different PM. When burning B0, the PM contour in the overlapping area is clear and relatively easy to distinguish. When burning B20 and B50, the PM contour in the overlapping area is gradually blurred and difficult to distinguish, appearing more irregular shapes, such as bulk, chain etc. When burning B100, inclusions are appeared on some PM surfaces, and the profile is approximately elliptical. With the increase of the biodiesel mixing ratio, the content of VOF on the surface of PM is increased, so that the PM contour in the overlapped area is not obvious due to the coverage of VOF. According to the microscopic morphology of PM, 150 spherical like PM were selected to calculate the PM diameter distribution, and the results is shown in Fig. 5. It can be seen that with the increase of biodiesel mixing ratio, the average PM diameter is decreased gradually. the average PM diameter of diesel is 26.27 nm, and B20, B50, B100 is decreased by about 7.9%, 17.2% and 24.2% respectively compared with B0. There are maybe three reasons: (1) biodiesel as oxygenated fuel will
biodiesel adsorbs more liquid substance. However, the PM diameter of other biodiesel fuels are smaller than that of diesel by about 7–24 nm. On the one hand, because of the PM mutual accumulation, reunion in the exhaust channel, the PM diameter is increased, on the other hand, PM will be cooled during the collection process. So VOF such as hydrocarbons in the exhaust gas will be condensed and adsorbed on the surface of PM, which will increase the PM diameter. 4.2. Microstructure and component of PM 4.2.1. Morphology Fig. 4 is the TEM photos of PM collected in 186F diesel engine fueled with diesel/waste cooking oil biodiesel blend. As can be seen from Fig. 4, the PM aggregate emitted by the diesel engine are deposited by several approximately spherical PM under the action of thermophoretic force and Van der Waals forces. PM is mainly formed in two ways. One way is the oxidation and pyrolysis products of fuel molecules, in which the PM diameter is generally greater than 100 nm. The other way is nucleation of the supersaturated sulfur vapor and unburned hydrocarbons, in which the PM diameter is generally smaller than 50 nm. When burning B0, the micro morphology of PM emitted by diesel engines is annular. With the increase of biodiesel mixing ratio, the microcosmic morphology of PM develops to cluster. Compared with
Fig. 3. PM diameter distribution of six kinds of biodiesel PM.
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Fig. 4. TEM photos of diesel/biodiesel PM samples.
energy level of π∗ or σ∗, resulting in a transition phenomenon. There are many types of functional groups on the surface of PM, and the near edge energy is generally distributed in the range of 280–300 eV. The energy transition range of 1s → π∗ transition, for the most functional groups, is 284–291 eV, and the transition from 1s → σ∗ is over 292 eV. Fig. 7 shows a NEXAFS curve of PM for diesel/biodiesel blends. It can be seen that the spectral lines of the blends with different mixing ratio are basically similar, and the energy range corresponding to the absorption peak is basically the same. This indicates that the functional groups of PM formed in different conditions are similar. However, there are obvious differences in the absorption peak strength, which indicates that the content of functional groups on the surface of PM varies with the mixing ratio of biodiesel. The absorption peaks of the soft X-ray absorption spectrum can be related to the chemical bonds and valence states of the carbon atoms in PM [34]. The experimental curves for all PM samples have obvious absorption peaks at 285.5 eV, 286.6 eV, 286.8 eV, 287.2 eV, 288.4 eV and 282.9 eV, corresponding to the “graphite” C]C π∗ formants, phenolic CeOH π∗ formants, ketone C]O π∗ formants, aliphatic CeH 3p/π∗ formants, carboxyl C]O π∗ formants and aliphatic C ] C σ∗ formants. With the increase of the biodiesel mixing ratio, the light absorption coefficients at 284.7–287.0 eV and 291.0–300.0 eV is increased gradually, and the absorption coefficients at 287.1–290.6 eV is decreased gradually. It is shown that the content of “graphite” C]C, phenols CeOH, ketones C]O and aliphatic C]C in PM have been decreased gradually, and the contents of aliphatic CeH and carboxyl C]O have been increased gradually.
Fig. 5. Average PM diameter of diesel/biodiesel.
increase the oxygen concentration in the combustion process. Therefore, the hydrocarbon and other organic substances on the surface of PM are easy to consume by oxidation; (2) the average PM diameter of biodiesel is smaller than diesel and the specific surface area is larger. Because of that, in the process of PM formation, the contact area with oxidant such as O2 and OH is increased, and the oxidation ability is increased as well; (3) as the cetane number of biodiesel is slightly higher, the premixed combustion period is shorter, and the diffusion combustion period is longer. This results in an increasing of PM oxidation time.
4.2.4. Component Fig. 8 is a TG-DTG curve for PM emissions from diesel and biodiesel emissions. As can be seen from Fig. 8, the TG curve of PM has two distinct stages of mass loss. The first mass loss stage is mainly the precipitation, evaporation, thermal pyrolysis for VOF in PM, and the corresponding temperature range is about 403–623 K. The second stage is mainly for combustion reaction of soot in PM, and the corresponding temperature range is about 673–923 K. In the first mass loss stage, with the increase of biodiesel mixing ratio, the content of VOF in PM and the peak value of mass loss rate are increased. In the second mass loss stage, with the increase of the biodiesel mixing ratio, the content of soot in PM and the peak value of mass loss rate are decreased, and the final residue in PM is increased. The main reasons for the increase of metal content in PM are high kinematic viscosity, slow flame propagation rate and long combustion duration of biodiesel. PM of diesel engine contains a small amount of water, VOF, soot, metal inorganic salts and other components. Due to the different boiling and ignition points of the substances contained in PM, the substance content in PM can be determined by the percentage of PM mass loss in different temperature ranges by using the thermogravimetric test. According to the mass loss percentage of PM in different temperature ranges, the relative content of each component in PM is obtained, and the result is shown in Fig. 9. With the increase of the biodiesel blending ratio, water and VOF content are increased. For example, water and VOF content of B0, B20, B50, B100 PM were 11.8%, 15.5%, 17.4%, 26.6%, respectively. With the increase of biodiesel blending ratio, the
4.2.2. Elements Fig. 6 shows the X-ray energy spectra of diesel/biodiesel PM. Each element in PM corresponds to the corresponding energy range on the X axis. To determine the elements of the PM, some related Refs. [30–33] have been accepted. According to the peaks in X ray energy spectrometer, the elements in PM can be determined. As can be seen from Fig. 6, The PM emitted from diesel engine, mainly contains C, O, Na, Cu, Al, Si, P, K, Ba, Zn and other elements. In these elements, C is the main element of PM, O is from incomplete oxide of hydrocarbon fuel, Na, Cu, Si, K, Be and Zn are from lubricating oil and Al is come from abrasion of engine parts, such as piston. S could be detected in the PM of B0, B20, and B50, however, the X ray intensity is decreased gradually, even to the extent that S is not detected in PM of B100. With the increase of biodiesel mixing ratio, the X ray intensity of C in PM is decreased gradually, and the X ray intensity of O, Na, Cu, Al, Si, Ba and Zn are increased gradually, indicating that oxygen-containing substance content in PM is increased, and the metal adsorbed in lubricating oil is increased. 4.2.3. Functional group During NEXAFS test, the electron, which is in outermost s orbit of atom in PM, can go into excited status by excitation into a higher 223
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(a)B0, (b) B20, (c) B50, (c) B100 Fig. 6. X ray energy spectrometer of diesel/biodiesel PM samples.
PM. The methyl functional group in biodiesel can reduce the precursors of PM such as C2H2, and inhibit the formation of PM, so that the content of metal was increased relatively. In order to evaluate the oxidation characteristics of PM, three characteristic temperatures were selected: ignition temperature Ti, peak temperature of mass loss rate Tm, and burnout temperature Tb. Where Ti is the temperature of PM mass loss rate at −0.1%·K−1 in the second mass loss stage, Tm is the temperature of PM mass loss rate peak in the second mass loss stage and Tb is the temperature of PM mass loss rate at-0.1%·K−1 in the end of the oxidation reaction. According to the thermogravimetric curve, the apparent activation energy of PM was calculated by Coats-Redfern method [15]. Table 3 shows the ignition temperature Ti, the peak temperature of mass loss rate Tm, as well as the burnout temperature Tb and the apparent activation energy E of PM for the diesel/biodiesel blend. With the increase of the biodiesel mixing ratio, Ti of PM is decreased gradually. Compared with B0, Ti of PM for B20, B50 and B100 emission is decreases by approximately 7.4 K, 14.6 K and 26.8 K, respectively. The results show that Ti of PM is decreased and the required ignition energy for PM is also decreased, which is benefit for the reduction of regeneration temperature of DPF. Tm of PM for B0 and B20 is pretty close, which is about 901 K. Tm of PM for B50 and B100 is decreased obviously, compared with that of B0, which is decreased by about 11.4 K and 27.3 K respectively. When the mass fraction of biodiesel in the blend is higher than 50%, the PM diameter is reduced significantly with the addition of biodiesel and the specific surface area is increased. In the process of thermal gravity, the contact area between PM and oxygen is increased, resulting in the decrease of Tm. Tb of PM has a consistent trend with Tm. This indicates that as soon as the biodiesel mixing ratio reached to 50%, the combustion rate of PM gets faster, and the regeneration time of DPF can be reduced. From Table 3 it can also be seen that with the increase of the biodiesel content in the blend, the apparent activation energy required for PM oxidation is decreased. According to the analysis of the PM
Fig. 7. NEXAFS spectra of PM samples for diesel/biodiesel blend.
content of soot in PM is decreased gradually. For example, compared with B0, soot in PM of B20, B50 and B100 is decreased by 3.8%, 6.6% and 17%, respectively. In the later stage of high temperature mass loss, with the increase of temperature, the quality remained is no long changed, because there is still some material remained in the crucible. The material remained is mainly some non-combustible substances such as metal inorganic salt. With the increase of the biodiesel mixing ratio, the metal inorganic salt content in PM is increased gradually. Compared with diesel, the higher viscosity of biodiesel has caused an increase of blend viscosity, and a deterioration by the breakup of droplets, atomization of the spray and diffusion. PM are easy to absorb the unburned biodiesel and intermediates such as HC, which the VOF content is increased in PM. Besides, the combustion duration is extended with the addition of biodiesel and the adsorption time of metal in lubricating oil by PM is increased. The result is the increase of metal inorganic salts in 224
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(a) TG
(b) DTG Fig. 8. TG and DTG curves of diesel/biodiesel PM samples.
PM were analyzed. Taking the waste cooking oil biodiesel as the research object, the PM of diesel engine fueled with an diesel/waste cooking oil biodiesel blend of 0%, 20%, 50%, 100% biodiesel mixing ratio were collected. The effects of biodiesel on morphology, PM diameter, elements, functional groups and component of PM were analyzed. The conclusions are as following: (1) The PM of biodiesel from different raw materials are mostly spherical, the average diameter is between 20 and 90 nm. The PM diameter of biodiesel made from cottonseed is larger than that from other different sources. (2) With the increase of biodiesel blend mixing ratio from 0% to 20%, 50% to 100%, the diameter of PM is decreased by 7.9%, 17.2% and 24.2% compared with that of diesel respectively and the microscopic morphology of PM changes from ring to cluster. (3) The X ray energy spectrometer analysis shows that, with the increase of biodiesel blend mixing ratio, the content of C in PM is reduced, however, the content of O, Na, Cu, Al, Ba and Zn is increased. The NEXAFS analysis show that the increase of biodiesel mixing ratio results in the reduction of “graphite” C]C, phenol CeOH, ketone C]O, aliphatic C]C in PM and the increase of aliphatic hydrocarbon CeH, carboxyl C]O in PM. (4) With the increase of the biodiesel blend mixing ratio, VOF in PM is increased and soot in PM is decreased. When the biodiesel blend mixing ratio reaches 50%, Ti, Tm, Tb and E of PM are decreased obviously, it means that oxidation of PM was more actively.
Fig. 9. Contents of each component in PM of B0, B20, B50.
Table 3 Ti, Tm, Tb and E of PM samples from combustion of diesel and biodiesel. Fuel
Ti /K
Tm /K
Tb /K
E/(kJ·mol−1)
B0 B20 B50 B100
772.9 765.5 758.3 746.1
901.7 900.3 890.3 874.4
927.9 926.4 922.0 912.4
68.7 61.3 57.8 48.5
Acknowledgement functional groups and elements, it can be seen that the addition of biodiesel causes an increase of aliphatic CeH and carboxyl C]O, Na, Cu, Al, Si, Be and Zn in PM. Wang’s study showed that aliphatic CeH had better effects on promoting the oxidation activity of PM than that of CeO [35]. Liati’s study showed that carbon material containing with carboxyl or carbonyl functional groups required lower energy to react and produce CO or CO2, making the carbon atoms free from their original position [36]. In addition, Seong’s study showed that Zn, Si, Fe, Ca and other elements in PM played a catalytic role in the oxidation process and reduced the energy required for PM oxidation [37].
This study was financially supported by the National Natural Science Foundation of China under Grant (No. 51776089). References [1] http://auto.ifeng.com/pinglun/20170909/1096087.shtml, in. [2] Ramalingam S, Rajendran S, Ganesan P, Govindasamy M. Effect of operating parameters and antioxidant additives with biodiesels to improve the performance and reducing the emissions in a compression ignition engine – A review. Renew Sustain Energy Rev 2018;81:775–88. [3] Shehata MS. Emissions, performance and cylinder pressure of diesel engine fuelled by biodiesel fuel. Fuel 2013;112:513–22. [4] Shameer PM, Ramesh K. Assessment on the consequences of injection timing and injection pressure on combustion characteristics of sustainable biodiesel fuelled engine. Renew Sustain Energy Rev 2018;81:45–61. [5] An H, Yang WM, Maghbouli A, Li J, Chou SK, Chua KJ. Performance, combustion and emission characteristics of biodiesel derived from waste cooking oils. Appl Energy 2013;112:493–9. [6] Nabi MN, Zare A, Hossain FM, Ristovski ZD, Brown RJ. Reductions in diesel
5. Conclusions In this paper, the PM of diesel and 5 kinds of biodiesel made from different sources were collected under atmospheric environment, and the effects of biodiesel raw materials on morphology and diameter of 225
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Full Length Article
Study on status characteristics and oxidation reactivity of biodiesel particulate matter
T
⁎
Ruina Li , Zhong Wang School of Automobile and Traffic Engineer, Jiangsu University, Zhenjiang 212013, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Diesel engine Biodiesel PM Status characteristics Oxidation reactivity
Biodiesel is a kind of clean and renewable fuel for diesel engine. The source of biodiesel affects the component and physicochemical properties of biodiesel, and it also influences the formation of particulate matter (PM) during the combustion process. In this paper, the PM of diesel and 5 kinds of biodiesel from different sources were collected. In addition, the effects of biodiesel from different sources on morphology and PM diameter were analyzed. Taking the biodiesel made from waste cooking oil as the research object, the PM of diesel engine fueled with diesel/biodiesel blend ratio of 0%, 20%, 50%, 100% were collected by the test bench at engine speed 2000 r/min and engine torque 12.27 N·m. The effects of biodiesel on morphology, diameter, elements, functional groups and component of PM were analyzed. Results show that the average PM diameter of biodiesel from different sources is between 20 and 90 nm. The PM diameter of biodiesel made from cottonseed is larger than that from other different sources. From the test, the average PM diameter of diesel is 26.27 nm. With the increase of biodiesel blend mixing ratio from 0% to 20%, 50% to 100%, the diameter of PM is decreased by 7.9%, 17.2% and 24.2% compared with that of diesel respectively and the microscopic morphology of PM changes from ring to cluster. The analysis of X ray energy spectrometer shows that the content of C in PM is reduced. However, the content of O, Na, Cu, Al, Ba and Zn is increased. The analysis of X-ray absorption near edge structure shows that the content of “graphite” C ] C, phenol CeOH, ketone C ] O, aliphatic C]C in PM is decreased gradually, and the content of aliphatic hydrocarbon CeH and carboxyl C]O is increased. With the increase of biodiesel blend mixing ratio, the content of volatile organic compounds (VOF) is increased and the content of soot is decreased. When biodiesel mixing ratio reaches 50%, Ti, Tm, Tb and E of PM are decreased obviously from thermo-gravimetric analyzer test, it means that oxidation of PM was more actively.
1. Introduction
Compared with diesel, biodiesel can effectively reduce the emission of unburned hydrocarbons (HC), carbon monoxide (CO) and PM in diesel engine [2–6]. PM is mainly composed of VOF, soot and metal components. The component and structure of PM have a great influence on the oxidation activity of PM, and the oxidation activity of PM influences on the regeneration of diesel particulate filter (DPF) carrier. For the PM with higher oxidation activity, the regeneration temperature is lower and the regeneration efficiency is higher [7]. Biodiesel made from different raw materials contains different types of esters, and the physicochemical properties will be different from each other, leading to different combustion parameters of biodiesel and different oxidation activity of PM. Abdalla [8] studied the properties for diesel, biodiesel and blended fuels. The results indicated that a linear increase with increasing the biodiesel percentage in the blend. Sulphur content decreased continuously with increasing the biodiesel percentages in the blend while the acid content increased with increasing biodiesel ratio. Madheshiya [9] studied the soot emissions of diesel
In 2016, China's dependence on foreign oil has reached 65.4% [1]. Finding clean alternative fuels for diesel engines has become an important topic among scholars. Biodiesel is prepared by transesterification of vegetable oils or animal fats. The raw materials of biodiesel don’t contain sulfur, aromatic compounds and other harmful substances. Diesel engine fueled with biodiesel fuel could reduce particulate matter (PM) emission, and it can relieve haze and air problems. Besides, the degradation rate of biodiesel is as high as 98%, which is 2 times of that of mineral diesel. It is a kind of high quality green environmental protection renewable energy [4–7]. In China, waste cooking oil is the main source for biodiesel. At present, Chinese biodiesel has achieved large-scale production and increased year by year. In 2016, China had more than 50 biodiesel product companies with a total capacity of more than 3.5 million tons. Biodiesel can be mixed with diesel or used directly in diesel engines. ⁎
Corresponding author. E-mail address: [email protected] (R. Li).
https://doi.org/10.1016/j.fuel.2018.01.041 Received 7 November 2017; Received in revised form 2 January 2018; Accepted 11 January 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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Fe, Cr, Cu, Mn, Pb, Ni, Ba and Cd) and inorganic anions (such as nitrates, chlorides, fluorides and sulphates) were compared for biodiesel/ diesel blend and diesel. The results showed that biodiesel origin particulate contained slightly lower amount of trace metals and inorganic ions compared with diesel for similar operating conditions except nitrates. According to the analysis above, biodiesel has a wide range of sources and complex components, and it will influence the formation of PM from diesel engine. The oxidation activity of PM is closely related to the microstructure, such as elements, component, and functional group. However, the PM oxidation activity has most important effect on exhaust emission of the engine, and it affects after-treatment of diesel engine. There are no enough reports to explanation the status characteristics and oxidation activity of biodiesel PM. From this view points of biodiesel sources and fuel characteristics, it is necessary to carry out the research on the status characteristics and oxidation reactivity of biodiesel PM. Friendly environment and stricter engine emission stander requires lower diesel engine particulate emissions. DPF is one of the effective way to reduce the PM. With the time going on, the PM adsorbed in the DPF will cause problems such as blockage of the DPF, etc. Performance of diesel engine will be reduced. This involves the problem of DPF regenerated. The objective of this research is to study the influence of biodiesel on the status characteristics and oxidation reactivity of PM. In this paper, the PM diameter distribution of diesel oil and biodiesel made from five different sources were investigated by the combustion test at normal temperature and pressure. Then the waste cooking oil biodiesel was selected to add into diesel, and PM of diesel/biodiesel blend with different blending ratio were collected on engine bench test. By means of high resolution transmission electron microscopy (TEM), X ray energy spectrometer, thermogravimetric analyzer and X-ray absorption near edge structure (XANES) test, the micro morphology, element type, chemical component and functional group of the PM were measured, and the effect of biodiesel on the formation of PM characteristics was discussed.
engine fueled with diesel, waste cooking oils and mustered oil. The results showed that soot emissions were alike for diesel, waste cooking oils, and mustered oils at low load, but at higher load diesel had an exponential increment in soot emissions. Zhao [10] studied analysis of fuel structures on engine soot particles’ mass and size using diesel and different levels of unsaturated biodiesel fuels through the numerical simulation. The results showed that the biodiesel fuel with a higher fraction of unsaturated fatty acid methyl esters (more double carbon bonds C]C) contributed more to the formation of soot precursors, thus producing a higher amount of soot particles in mass and numbers, accelerating soot particle nucleation and soot surface growth. Giakoumis [11]studied the effects of diesel/biodiesel blends on the exhaust emissions of diesel engines operating under transient conditions. The results showed that the use of biodiesel blends, regardless of their source material, resulted in dramatic PM mass reductions ranging from 67% to 74%. These reductions could be attributed to the increased oxygen concentration in the biodiesel blend, which reduced locally fuel-rich regions and limits soot nucleation in the formation process. Grigoratos [12]tested diesel as well as three blends of rapeseed methyl ester at 10%, 30% and 50% v/v with three Euro 4+ compliant vehicles. The result showed higher oxidative activity with increasing biodiesel blending when testing two light-duty vehicles with and without a DPF over the NEDC and Artemis cycles. Agudelo etc. [13] compared the oxidation activity of PM produced by diesel, palm oil biodiesel and jatropha oil biodiesel. The results showed that PM oxidation activity of Jatropha curcas oil biodiesel was the highest, followed by palm oil biodiesel, and diesel. Karavalakis [14] assessed PM on two heavy-duty trucks fueled with ultra-low sulfur diesel and biodiesel blends as well as the oxidative potential of the PM measured by means of the dithiothreitol assay. The results showed a reduction on oxidative potential of PM with the use of biodiesel blends relative to ultra-low sulfur diesel. The oxidation activity of PM is closely related to the microstructure of PM. Disordered carbon atoms, which are located in PM microcrystalline graphite layer, have unpaired sp2 electrons, are more easily to form a covalent bond with the chemical adsorption oxygen. However, the ordered carbon atoms in microcrystalline graphite layer only with sharedπelectron, are difficult to combine with oxygen. The surface functional group on the edge of the PM carbon layer is more likely to react with the oxidized gas to form CO or CO2, and to make the carbon atoms fall off the original chemical bond. Cain etc. [15]studied the distribution of functional groups on soot PM. The results showed that many types of functional groups were present on the surface of the soot PM, including aliphatic CeH functional group, aromatic CeH functional group and oxygen-containing group (C]O, CeOOH, CeOH, CeOeC). Among these functional groups, the relative content of aliphatic CeH functional group was the highest. Nabi [6]studied PM and particulate number (PN) of biodiesel/diesel blends in a six-cylinder turbocharged diesel engine with a high-pressure common rail injection system in compliance with a 13-Mode European Stationary Cycle. Results showed that with the addition of biodiesel, a significant reduction in both PM and PN emissions was observed relative to those of diesel. A maximum reduction of 84% PM and 88% PN was observed by using the biodiesel blends. Wei [16] studied the influence of waste cooking oil biodiesel on particulate emissions of a direct-injection diesel engine. The results showed that the addition of biodiesel reduced the weighted particle mass concentration and the weighted geometric mean diameter of the particles. The influence of biodiesel on the investigated emissions was proportional to the biodiesel content in the tested fuels. Gali [17] studied the chemical components of solid and semi-volatile PM from diesel and biodiesel blend. The results showed that PM emission was lower for biodiesel/diesel blend (30% biodiesel added) in general. PM for biodiesel/diesel blend contained 19% higher organic compounds in semi-volatile PM compared with diesel, which on the other hand noted 24% more solid PM. Shukla [18] studied the trace metals and ions in particulates of engine fueled with Karanja biodiesel blend (20% v/v biodiesel in the blend fuel), and 9 commonly present trace metals (Ca,
2. Analysis of biodiesel quality and PM formation factors 6 kinds of fuel were used in this test, including 0# diesel, waste cooking oil biodiesel, rapeseed oil biodiesel, soybean oil biodiesel, cottonseed oil biodiesel and palm oil biodiesel. 0# diesel is commercial diesel, soybean oil biodiesel is produced by Hainan positive and Bio Energy Limited, waste cooking oil biodiesel is produced by Jiangsu Carter Petroleum New Energy Co., Ltd. The remaining 3 kinds of biodiesel are prepared in Jiangsu biodiesel power machinery application engineering center. The physicochemical parameters of the fuels are listed in Table 1. The formation mechanism of PM is complex, which is greatly affected by the combustion process. It is generally believed that soot size and amount of formation are related to temperature, pressure, time, oxygen content, fuel properties and so on. Local high temperature, hypoxia, cracking and dehydrogenation are benefit for the generation of PM. The carbon structure of PM has an important effect on the oxidation of PM. The PM whose carbon atoms are arranged orderly, like in a graphite structure, are difficult to be oxidized in the combustion process [19]. Thus, the properties of the fuel which will influence the formation of PM are analyzed below. 2.1. Oxygen content The area in front of the burning flame is rich of oxygen, and soot is mainly generated on the side of the flame front. The average oxygen content in biodiesel is around 11%. The oxygen in biodiesel can improve the situation of oxygen deficiency and promote combustion. So, combustion condition of biodiesel is better compared with diesel and less soot is formed. Bedjanian’s study showed that fuel quality played an important role in the formation of PM. Oxygen in the fuel could 219
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Table 1 Physicochemical properties of the six fuels. Parameters
0# diesel
Waste cooking oil biodiesel
Rapeseed oil biodiesel
Soybean oil biodiesel
Cottonseed oil biodiesel
Palm oil biodiesel
ρ20/(kg/m3) V40/(mm2/s) Cetane number w(S)/% w(C)/% w(H)/% w(O)/% Lower heat value /(kJ/kg)
830 2.4 45 0.2 87.0 12.6 0.4 42.5
877.5 4.1 52 0.0007 77.1 12.2 10.1 37.99
876.4 4.0 54.4 0.001 68.9 10.6 10.9 38.61
881 4.47 51.2 0.001 67.89 7.77 13.5 37.27
880 3.923 58 0.007 78.1 11.5 10 38.13
874 4.38 58.3 0.004 75.6 13 12.8 37.1
resulted in a smaller amount of PM generated by biodiesel. Labeckas carried out a diesel engine test with ethanol and biodiesel, and studied the effects of the cetane number on exhaust smoke. The results showed that smoke can be decreased by 1.7 times or by 34.9%, when running the engine with the higher cetane number (67.3) blend, compared with a lower cetane number blend [25]. Vallinayagam’s study showed that fuel with higher cetane number leaded to less soot [26].
significantly inhibit the nucleation of PM, and the increase in oxygen content could reduce soot volume fraction and PM size [20]. Ramin tested the PM oxidation characteristics of diesel engine by using a thermogravimetric analyzer. The results showed that the microstructure of PM and the oxygen-containing groups had great influence on the oxidation activity of PM. The more regular the PM structure was, the less was the number of oxygen groups and the lower was the activity of oxidation [21]. Wall launched a study on the microstructure and oxidation characteristics of PM carried out on a diesel engine fueled with oxygen added diesel fuel. The results showed that the addition of oxygen into the fuel leaded to an larger PM layer distance and curvature, but a smaller crystallite size [22].
3. Experimental scheme and equipment 3.1. Experimentation methodology (1) In order to study the effects of biodiesel sources on the PM formation, the PM samples of diesel, waste cooking oil biodiesel, rapeseed oil biodiesel, soybean oil biodiesel, cottonseed oil biodiesel, palm oil biodiesel were collected in an atmospheric combustion test. The micro morphology and PM diameter were analyzed through TEM. (2) As biodiesel in China is mostly made from waste cooking oil biodiesel, in order to investigate the effects of biodiesel on the microstructure and component characteristics of PM, a diesel engine bench test was carried out to collect PM samples of diesel/ waste cooking oil biodiesel blend. The experiment was carried out in the 186F single cylinder direct injection diesel engine. A scheme of the test engine set up is shown in Fig. 1 and the specifications of the test engine are provided in Table 2. The experiment was carried on a stable diesel engine, running under the condition of 2000 r/min and 12.27 N·m. The biodiesel mixing ratios of experimental fuel were 0%, 20%%, 50% and 100%, recorded as B0, B20, B50 and B100, respectively. The micro-orifice uniform deposition impactor (MOUDI) was used to collect PM of the blends. During the sampling, the standard aluminum foil filter paper (47 nm × 8, MSP) was used to collect PM samples. The sampling flow was 30 L/min, and the sampling time was 20 min. In order to reduce the
2.2. Aromatic hydrocarbon content Aromatic hydrocarbon, which includes single ring, double rings as well as a small number of tricyclic structure, is difficult to evaporate, mix and combust. At higher temperature, the ring structure of aromatic hydrocarbon does not prone to cleavage, but it occurs condensation directly and generate soot precursors of polycyclic aromatic hydrocarbons. Therefore, the more aromatic hydrocarbon is contented, the greater is the amount of generated soot. The mass fraction of aromatic hydrocarbons in diesel oil is about 5.82%-21.39%. The main component of biodiesel is fatty acid, which contains almost no aromatic hydrocarbon. Therefore, the amount of soot, generated by biodiesel is smaller than that of diesel fuel. Xie integrated a skeletal polycyclic aromatic hydrocarbon model with a toluene reference fuel oxidation model. The results show that the predicted benzene concentration in the oxidation of alkanes and aromatic hydrocarbons indicates that the molecular structure of the fuel significantly affects the PAH formation pathway [23]. 2.3. Sulfur content Sulfur content which is an important indicator of oil products, is directly related to the PM amount of diesel engine. The increase of sulfur content of diesel results in an increase of sulfate in the combustion process. The higher sulfur concentration on the soot surface increases the PM size. In addition, the intermediate product of sulfur reactions can promote soot formation and increase the total number of PM. In China, the national standard V for vehicle diesel sulfur content limit is 10 mg/kg. As biodiesel is made from vegetable oil and animal fats, there is no sulfur in biodiesel. So, the PM size of biodiesel is smaller than that of diesel and the amount of PM is less than that of diesel. 2.4. Cetane number
1- Eddy current dynamometer, 2- Diesel engine, 3- Optical encoder, 4-Pressure sensor, 5-Smoke intensity, 6- Exhaust analyzer, 7- Cooler, 8- Float flowmeter, 9- Interial impactor, 10- Up pressure gauge, 11- Down pressure gauge, 12- Flow valve, 13- Pump
Cetane number is a measure of the ignition performance of diesel fuel. The cetane number of the fuel is high, the ignitability is better. Cetane number also affects the emissions of the diesel engine. The research showed that increasing the cetane number of diesel fuel could reduce CO, HC and soot [24]. The Cetane number of 0# diesel is about 45, the cetane number of biodiesel is between 51 and 59, which
Fig. 1. Schematic of test engine set up.
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before (I0) and after (I1) sample was obtained. Then the light absorption coefficient could be obtained according to the formula μ(E) = ln(I1/I0). The PM functional groups of B0, B20, B50 and B100 were measured.
Table 2 Engine specifications of 186F. Type
Direct-injected, 4 stokes, air-cooled, natural aspiration
Number of cylinders Cylinder bore (mm) × stroke (mm) Compression ratio Rated power (kW)/speed(r/min) Maximum power(kW) /speed (r/min) Nozzle number × orifice diameter (mm) Injection start (°CA BTDC)
1 86 × 70 19 5.7/3000 6.9/1800 4 × 0.24 12
3.2.4. Thermogravimetric analyzer The thermogravimetric analysis was used to test the quality of the sample in the heating furnace with time and temperature. The experiment was carried out on TGA/DSC1 thermogravimetric analysis produced by Mettler-Toledo in Switzerland. Temperature range: room temperature to 1600 °C, Temperature accuracy of ± 0.3 °C, heating rate 0.1–150 °C/min, Sample load 0–1000 mg, Balance sensitivity 0.1 μg, temperature resolution 0.0001 °C. The particle sample is weighed by the MX5 type micro-electronic balance of Mettler-Toledo company, Switzerland. Weighing range 0–5.1 g, minimum score 1 μg. The combustion temperature ranges of volatile matter and soot are different for diesel engine combustion particles. Therefore, through the thermogravimetric test, volatile substances and soot can be distinguished. During the experiment, the oxidation gas was O2 and the protective gas N2. The oxidation gas and the protective gas flow rate was 50 L/min, the heating rate was selected at 20 K/min, the temperature interval was 20–750 K and the initial mass of the PM was about 2 mg.
measurement error, multiple measurements were carried out at the same working condition to calculate the average values. Meanwhile, the PM collected at different working conditions were prepared for TEM, X ray spectrum, thermalgravimetricr and XANES analysis. 1- Eddy current dynamometer, 2- Diesel engine, 3- Optical encoder, 4- Pressure sensor, 5- Smoke intensity, 6- Exhaust analyzer, 7- Cooler, 8- Float flowmeter, 9- Interial impactor, 10- Up pressure gauge, 11Down pressure gauge, 12- Flow valve, 13- Pump
4. Results and discussion
3.2. Experimental equipments
4.1. Effects of biodiesel sources on PM microstructure
3.2.1. TEM The experiment was carried out on JEM-2100 (HR) high resolution transmission electron microscopy. The acceleration voltage was 200 kV, the point resolution 0.24 nm, the lattice resolution 0.14 nm, and the magnification 2000–1,500,000 times. Before the test, 1 mg of the PM was taken and placed in a centrifuge tube. A small amount of methylene chloride was dripped into the centrifuge tube, and then the centrifuge tubes were subjected to ultrasonic treatment for 20 min. After dropping 5 drops of ultrasound solution on the HRTEM copper mesh (230 mesh), bake the copper mesh under a hot lamp. Then the suitable acceleration voltage, condenser current, working distance, objective diaphragm and scanning speed were selected to ensure the image can satisfy the requirement of study.
The formation of PM undergoes nucleation, surface growth and condensation, agglomeration, and oxidation. After the formation of “core”, on the one hand, the gas phase component moves to the core surface, on the other hand, it is the collision and condensation between the core PM, so that the PM grows up and becomes approximately spherical. Fig. 2 shows the PM photograph of biodiesel from six different sources with a magnification of 50,000 times. The particle morphology shows different distribution characteristics of particle morphology. It can be seen that the basic PM are mostly spherical or ellipsoidal, where each of them are containing hundreds of quasi spherical basic PM of unequal size. These basic carbon PM accumulates under the adhesion force of Van der Waals force, electrostatic force, liquid bridging force and so on, to form different PM groups with different tightness [27,28]. In terms of density, the arrangement of PM in biodiesel is relatively close, and the PM in diesel oil is arranged loosely. According to the degree of caking, the PM of biodiesel are stronger, and they are mostly bonded, while the bonding of diesel PM is weak. Among the 5 kinds of biodiesel, the PM produced by waste cooking oil biodiesel has the smallest particle size and the highest agglomeration, and the PM produced by cottonseed oil biodiesel has the biggest particle size and the lowest agglomeration. The higher the unsaturated degree of the fuel molecules, the lower the cetane number, the worse the ignition, and worse ignition will lead to more and bigger PM. Study shows that the waste cooking oil biodiesel contains about 54.8% unsaturation fatty acid methylester, and cottonseed oil biodiesel contains about 60.3% unsaturation fatty acid methylester [29]. This maybe the reason why the PM produced by waste cooking oil biodiesel is smaller and the PM produced by cottonseed oil biodiesel is bigger. According to Fig. 2, a square contained about 20 particles in the TEM image was chosen randomly to measure the diameter of each particle. After getting the diameter for each particle, the average PM diameter could be calculated. For each TEM image, the statistics and calculation were repeated above 10 times. At last, the distribution of particle size and the final average PM diameter could be got. Fig. 3 shows the PM diameter of six kinds of fuel. It can be seen that the diameter of diesel and biodiesel PM is mainly distributed between 10 and 100 nm and most of them are between 20 and 90 nm. The fuel has a great influence on the average PM diameter and PM distribution. The PM diameter of biodiesel made from cottonseed oil is larger than that of diesel by about 4–17 nm. The main reason is that PM of cottonseed oil
3.2.2. X ray energy spectrometer Element species of PM were tested using a S-4800 field emission scanning electron microscope and a X ray energy spectrum analyzer, produced by Japan's Hitachik High-tech company. The electron emission source of the electron microscope or was cold field emission, and the objective lens was semi submerged. Two times electronic resolution was 1.0 nm (15 kV), 2.0 nm (1 kV), backscattered electron resolution was 3.0 nm (15 kV) and accelerating voltage was 0.5–30 kV (0.1 kV/ step, variable). The magnification was 30–800,000. The elemental analysis range of X ray spectrometer is Be4-U92. Before the test, a small amount of PM was placed into acetone solution and was treated with ultrasonic waves. After treatment the sample was placed on silicon wafers to dry, the samples were sprayed with metal by an ion sputtering apparatus. 3.2.3. XANES The XANES experiment of diesel engine PM was carried out in the BL08U1-A of Shanghai Synchrotron. The electron energy of storage ring was 2.5 Gev, flux was 150–300 mA, photon energy range 250–2000 eV, photon flux 106–109 (photons/s), the pressure of sample room is about 10–6 Torr, natural emissivity is 3.9 nm·rad and the energy resolution was greater than 1000. The sample frame can move in three dimensions and can be rotated around the vertical axis. The standard reference materials are oxalic acid, sucrose and graphite. During the experiment, the near edge X ray absorption spectrum of C on K edge was scanned and recorded by the step length of 0.2 eV, and the X ray intensity of 221
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Fig. 2. Photographs of six kinds of particulate (×50,000).
Diesel
Waste cooking oil biodiesel
Soybean oil biodiesel
Rapeseed oil biodiesel
Cottonseed oil biodiesel
Palm oil biodiesel
diesel, biodiesel has a greater viscosity. This will lead to a poorer droplet breakup, diffusion and atomization. The increase of unburned fuel and VOF will increase the number of nucleated PM, as well as the collision frequency and coagulation rate between PM, resulting in the microscopic morphology of PM develops to cluster. The PM aggregates show different tightness at the junction, where the deeper areas indicate a overlapping of different PM. When burning B0, the PM contour in the overlapping area is clear and relatively easy to distinguish. When burning B20 and B50, the PM contour in the overlapping area is gradually blurred and difficult to distinguish, appearing more irregular shapes, such as bulk, chain etc. When burning B100, inclusions are appeared on some PM surfaces, and the profile is approximately elliptical. With the increase of the biodiesel mixing ratio, the content of VOF on the surface of PM is increased, so that the PM contour in the overlapped area is not obvious due to the coverage of VOF. According to the microscopic morphology of PM, 150 spherical like PM were selected to calculate the PM diameter distribution, and the results is shown in Fig. 5. It can be seen that with the increase of biodiesel mixing ratio, the average PM diameter is decreased gradually. the average PM diameter of diesel is 26.27 nm, and B20, B50, B100 is decreased by about 7.9%, 17.2% and 24.2% respectively compared with B0. There are maybe three reasons: (1) biodiesel as oxygenated fuel will
biodiesel adsorbs more liquid substance. However, the PM diameter of other biodiesel fuels are smaller than that of diesel by about 7–24 nm. On the one hand, because of the PM mutual accumulation, reunion in the exhaust channel, the PM diameter is increased, on the other hand, PM will be cooled during the collection process. So VOF such as hydrocarbons in the exhaust gas will be condensed and adsorbed on the surface of PM, which will increase the PM diameter. 4.2. Microstructure and component of PM 4.2.1. Morphology Fig. 4 is the TEM photos of PM collected in 186F diesel engine fueled with diesel/waste cooking oil biodiesel blend. As can be seen from Fig. 4, the PM aggregate emitted by the diesel engine are deposited by several approximately spherical PM under the action of thermophoretic force and Van der Waals forces. PM is mainly formed in two ways. One way is the oxidation and pyrolysis products of fuel molecules, in which the PM diameter is generally greater than 100 nm. The other way is nucleation of the supersaturated sulfur vapor and unburned hydrocarbons, in which the PM diameter is generally smaller than 50 nm. When burning B0, the micro morphology of PM emitted by diesel engines is annular. With the increase of biodiesel mixing ratio, the microcosmic morphology of PM develops to cluster. Compared with
Fig. 3. PM diameter distribution of six kinds of biodiesel PM.
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Fig. 4. TEM photos of diesel/biodiesel PM samples.
energy level of π∗ or σ∗, resulting in a transition phenomenon. There are many types of functional groups on the surface of PM, and the near edge energy is generally distributed in the range of 280–300 eV. The energy transition range of 1s → π∗ transition, for the most functional groups, is 284–291 eV, and the transition from 1s → σ∗ is over 292 eV. Fig. 7 shows a NEXAFS curve of PM for diesel/biodiesel blends. It can be seen that the spectral lines of the blends with different mixing ratio are basically similar, and the energy range corresponding to the absorption peak is basically the same. This indicates that the functional groups of PM formed in different conditions are similar. However, there are obvious differences in the absorption peak strength, which indicates that the content of functional groups on the surface of PM varies with the mixing ratio of biodiesel. The absorption peaks of the soft X-ray absorption spectrum can be related to the chemical bonds and valence states of the carbon atoms in PM [34]. The experimental curves for all PM samples have obvious absorption peaks at 285.5 eV, 286.6 eV, 286.8 eV, 287.2 eV, 288.4 eV and 282.9 eV, corresponding to the “graphite” C]C π∗ formants, phenolic CeOH π∗ formants, ketone C]O π∗ formants, aliphatic CeH 3p/π∗ formants, carboxyl C]O π∗ formants and aliphatic C ] C σ∗ formants. With the increase of the biodiesel mixing ratio, the light absorption coefficients at 284.7–287.0 eV and 291.0–300.0 eV is increased gradually, and the absorption coefficients at 287.1–290.6 eV is decreased gradually. It is shown that the content of “graphite” C]C, phenols CeOH, ketones C]O and aliphatic C]C in PM have been decreased gradually, and the contents of aliphatic CeH and carboxyl C]O have been increased gradually.
Fig. 5. Average PM diameter of diesel/biodiesel.
increase the oxygen concentration in the combustion process. Therefore, the hydrocarbon and other organic substances on the surface of PM are easy to consume by oxidation; (2) the average PM diameter of biodiesel is smaller than diesel and the specific surface area is larger. Because of that, in the process of PM formation, the contact area with oxidant such as O2 and OH is increased, and the oxidation ability is increased as well; (3) as the cetane number of biodiesel is slightly higher, the premixed combustion period is shorter, and the diffusion combustion period is longer. This results in an increasing of PM oxidation time.
4.2.4. Component Fig. 8 is a TG-DTG curve for PM emissions from diesel and biodiesel emissions. As can be seen from Fig. 8, the TG curve of PM has two distinct stages of mass loss. The first mass loss stage is mainly the precipitation, evaporation, thermal pyrolysis for VOF in PM, and the corresponding temperature range is about 403–623 K. The second stage is mainly for combustion reaction of soot in PM, and the corresponding temperature range is about 673–923 K. In the first mass loss stage, with the increase of biodiesel mixing ratio, the content of VOF in PM and the peak value of mass loss rate are increased. In the second mass loss stage, with the increase of the biodiesel mixing ratio, the content of soot in PM and the peak value of mass loss rate are decreased, and the final residue in PM is increased. The main reasons for the increase of metal content in PM are high kinematic viscosity, slow flame propagation rate and long combustion duration of biodiesel. PM of diesel engine contains a small amount of water, VOF, soot, metal inorganic salts and other components. Due to the different boiling and ignition points of the substances contained in PM, the substance content in PM can be determined by the percentage of PM mass loss in different temperature ranges by using the thermogravimetric test. According to the mass loss percentage of PM in different temperature ranges, the relative content of each component in PM is obtained, and the result is shown in Fig. 9. With the increase of the biodiesel blending ratio, water and VOF content are increased. For example, water and VOF content of B0, B20, B50, B100 PM were 11.8%, 15.5%, 17.4%, 26.6%, respectively. With the increase of biodiesel blending ratio, the
4.2.2. Elements Fig. 6 shows the X-ray energy spectra of diesel/biodiesel PM. Each element in PM corresponds to the corresponding energy range on the X axis. To determine the elements of the PM, some related Refs. [30–33] have been accepted. According to the peaks in X ray energy spectrometer, the elements in PM can be determined. As can be seen from Fig. 6, The PM emitted from diesel engine, mainly contains C, O, Na, Cu, Al, Si, P, K, Ba, Zn and other elements. In these elements, C is the main element of PM, O is from incomplete oxide of hydrocarbon fuel, Na, Cu, Si, K, Be and Zn are from lubricating oil and Al is come from abrasion of engine parts, such as piston. S could be detected in the PM of B0, B20, and B50, however, the X ray intensity is decreased gradually, even to the extent that S is not detected in PM of B100. With the increase of biodiesel mixing ratio, the X ray intensity of C in PM is decreased gradually, and the X ray intensity of O, Na, Cu, Al, Si, Ba and Zn are increased gradually, indicating that oxygen-containing substance content in PM is increased, and the metal adsorbed in lubricating oil is increased. 4.2.3. Functional group During NEXAFS test, the electron, which is in outermost s orbit of atom in PM, can go into excited status by excitation into a higher 223
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(a)B0, (b) B20, (c) B50, (c) B100 Fig. 6. X ray energy spectrometer of diesel/biodiesel PM samples.
PM. The methyl functional group in biodiesel can reduce the precursors of PM such as C2H2, and inhibit the formation of PM, so that the content of metal was increased relatively. In order to evaluate the oxidation characteristics of PM, three characteristic temperatures were selected: ignition temperature Ti, peak temperature of mass loss rate Tm, and burnout temperature Tb. Where Ti is the temperature of PM mass loss rate at −0.1%·K−1 in the second mass loss stage, Tm is the temperature of PM mass loss rate peak in the second mass loss stage and Tb is the temperature of PM mass loss rate at-0.1%·K−1 in the end of the oxidation reaction. According to the thermogravimetric curve, the apparent activation energy of PM was calculated by Coats-Redfern method [15]. Table 3 shows the ignition temperature Ti, the peak temperature of mass loss rate Tm, as well as the burnout temperature Tb and the apparent activation energy E of PM for the diesel/biodiesel blend. With the increase of the biodiesel mixing ratio, Ti of PM is decreased gradually. Compared with B0, Ti of PM for B20, B50 and B100 emission is decreases by approximately 7.4 K, 14.6 K and 26.8 K, respectively. The results show that Ti of PM is decreased and the required ignition energy for PM is also decreased, which is benefit for the reduction of regeneration temperature of DPF. Tm of PM for B0 and B20 is pretty close, which is about 901 K. Tm of PM for B50 and B100 is decreased obviously, compared with that of B0, which is decreased by about 11.4 K and 27.3 K respectively. When the mass fraction of biodiesel in the blend is higher than 50%, the PM diameter is reduced significantly with the addition of biodiesel and the specific surface area is increased. In the process of thermal gravity, the contact area between PM and oxygen is increased, resulting in the decrease of Tm. Tb of PM has a consistent trend with Tm. This indicates that as soon as the biodiesel mixing ratio reached to 50%, the combustion rate of PM gets faster, and the regeneration time of DPF can be reduced. From Table 3 it can also be seen that with the increase of the biodiesel content in the blend, the apparent activation energy required for PM oxidation is decreased. According to the analysis of the PM
Fig. 7. NEXAFS spectra of PM samples for diesel/biodiesel blend.
content of soot in PM is decreased gradually. For example, compared with B0, soot in PM of B20, B50 and B100 is decreased by 3.8%, 6.6% and 17%, respectively. In the later stage of high temperature mass loss, with the increase of temperature, the quality remained is no long changed, because there is still some material remained in the crucible. The material remained is mainly some non-combustible substances such as metal inorganic salt. With the increase of the biodiesel mixing ratio, the metal inorganic salt content in PM is increased gradually. Compared with diesel, the higher viscosity of biodiesel has caused an increase of blend viscosity, and a deterioration by the breakup of droplets, atomization of the spray and diffusion. PM are easy to absorb the unburned biodiesel and intermediates such as HC, which the VOF content is increased in PM. Besides, the combustion duration is extended with the addition of biodiesel and the adsorption time of metal in lubricating oil by PM is increased. The result is the increase of metal inorganic salts in 224
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(a) TG
(b) DTG Fig. 8. TG and DTG curves of diesel/biodiesel PM samples.
PM were analyzed. Taking the waste cooking oil biodiesel as the research object, the PM of diesel engine fueled with an diesel/waste cooking oil biodiesel blend of 0%, 20%, 50%, 100% biodiesel mixing ratio were collected. The effects of biodiesel on morphology, PM diameter, elements, functional groups and component of PM were analyzed. The conclusions are as following: (1) The PM of biodiesel from different raw materials are mostly spherical, the average diameter is between 20 and 90 nm. The PM diameter of biodiesel made from cottonseed is larger than that from other different sources. (2) With the increase of biodiesel blend mixing ratio from 0% to 20%, 50% to 100%, the diameter of PM is decreased by 7.9%, 17.2% and 24.2% compared with that of diesel respectively and the microscopic morphology of PM changes from ring to cluster. (3) The X ray energy spectrometer analysis shows that, with the increase of biodiesel blend mixing ratio, the content of C in PM is reduced, however, the content of O, Na, Cu, Al, Ba and Zn is increased. The NEXAFS analysis show that the increase of biodiesel mixing ratio results in the reduction of “graphite” C]C, phenol CeOH, ketone C]O, aliphatic C]C in PM and the increase of aliphatic hydrocarbon CeH, carboxyl C]O in PM. (4) With the increase of the biodiesel blend mixing ratio, VOF in PM is increased and soot in PM is decreased. When the biodiesel blend mixing ratio reaches 50%, Ti, Tm, Tb and E of PM are decreased obviously, it means that oxidation of PM was more actively.
Fig. 9. Contents of each component in PM of B0, B20, B50.
Table 3 Ti, Tm, Tb and E of PM samples from combustion of diesel and biodiesel. Fuel
Ti /K
Tm /K
Tb /K
E/(kJ·mol−1)
B0 B20 B50 B100
772.9 765.5 758.3 746.1
901.7 900.3 890.3 874.4
927.9 926.4 922.0 912.4
68.7 61.3 57.8 48.5
Acknowledgement functional groups and elements, it can be seen that the addition of biodiesel causes an increase of aliphatic CeH and carboxyl C]O, Na, Cu, Al, Si, Be and Zn in PM. Wang’s study showed that aliphatic CeH had better effects on promoting the oxidation activity of PM than that of CeO [35]. Liati’s study showed that carbon material containing with carboxyl or carbonyl functional groups required lower energy to react and produce CO or CO2, making the carbon atoms free from their original position [36]. In addition, Seong’s study showed that Zn, Si, Fe, Ca and other elements in PM played a catalytic role in the oxidation process and reduced the energy required for PM oxidation [37].
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5. Conclusions In this paper, the PM of diesel and 5 kinds of biodiesel made from different sources were collected under atmospheric environment, and the effects of biodiesel raw materials on morphology and diameter of 225
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