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biodiesel blends
Renewable Energy 147 (2020) 1990e2002
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Investigation on fuel properties and engine performance of the extraction phase liquid of bio-oil/biodiesel blends Yongcheng Huang*, Yaoting Li, Xudong Han, Jiating Zhang, Kun Luo, Shangsheng Yang, Jiyuan Wang School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, 710049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 May 2019 Received in revised form 26 September 2019 Accepted 6 October 2019 Available online 7 October 2019
In this paper, biodiesel was used to upgrade bio-oil by extracting the high-quality fuel fractions in bio-oil through solvent extraction. After the extraction, the upper layer blends of biodiesel and the high-quality bio-oil components, also called the “extraction phase liquid”, was obtained and denoted as EPBB. Firstly, Thermogravimetric (TG) and Fourier transform infrared (FTIR) analyses indicated that the light components originally in bio-oil, such as alcohols, ethers, ketones and carboxylic acids were extracted from bio-oil to biodiesel and formed the EPBB. Then the fuel properties of EPBB were measured at different temperatures. The results showed that the density of EPBB was higher, while its viscosity, surface tension and lower heating value (LHV) were lower than those of biodiesel. Accordingly, EPBB revealed better atomization and evaporation characteristics as compared to biodiesel. Finally, the engine performance of EPBB was tested on an unmodified direct-injection diesel engine. When compared with biodiesel, the ignition delay of EPBB was longer, the premixed combustion duration was longer, while the total combustion duration was shorter. The fuel economy of EPBB was comparable to that of biodiesel, and the NOx, soot, CO and UHC emissions of EPBB were all lower than those of biodiesel. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Bio-oil Biodiesel Fuel property Diesel engine Combustion Emissions
1. Introduction With the gradual depletion of petroleum reserves and the widespread concerns about the environmental issues such as air pollution and greenhouse effect, there is a huge demand for vehicle engines, especially diesel engines to develop the clean and renewable alternative fuels. Liquid biofuels, which mainly refer to bioalcohols, biodiesels and bio-oils are regarded as promising alternative fuels for petroleum diesel. The superiority of biofuels is mainly attributed to their renewability and low emissions, especially in reducing the greenhouse gases (GHG) and defeating the smoke-NOx trade-off of fossil diesel [1e3]. Bio-oil, also known as biomass pyrolysis oil, is a dark brown, free flowing liquid fuel. It can be produced through the fast pyrolysis of a variety of renewable biomass resources such as waste wood, corn straw and rice husk [4]. Typically, it has high proportions of oxygen, water, acid and solids, high viscosity and surface tension, low heating value and unstable thermal properties [5]. Therefore, it is a
* Corresponding author. E-mail address: [email protected] (Y. Huang). https://doi.org/10.1016/j.renene.2019.10.028 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
kind of lower-grade fuel when compared with fossil fuels. However, bio-oil has several distinct advantages over fossil fuels on energy and environmental aspects. It has negligible contents of sulphur, nitrogen and ash, so it produces the lower Nitrogen Oxides (NOx) and Sulphur Oxides (SOx) emissions under most conditions when compared with conventional fossil diesel [6]. In addition, bio-oil is environmental-friendly since it is carbon neutral and biodegradable. Consequently, bio-oil has been recognized as a potential alternative fuel for transportation vehicles, and a large number of researches have been conducted on the application of bio-oil on diesel engines. Regrettably, bio-oil cannot ignite spontaneously without adding additives, and injector nozzle coking and clogging are also observed soon after the combustion of bio-oil [7]. Based on these facts, various methods such as adding additives [8], hot filtration [9] and diesel ignition improvers [10] were later proposed to promote the ignition and combustion of bio-oil in diesel engines. However, due to the poor properties of the crude bio-oil, the problems of fuel delivery system corrosion and injector nozzle clogging were still not solved by these methods. Therefore, in order to utilize bio-oil in diesel engines without much engine modification, an upgrade treatment of the crude bio-oil is necessary to improve its ignition
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Nomenclature
CA50
NOx SOx BTE TG FTIR CA CO DTG UHC
CA90
hm COV CA05
Nitrogen oxides Sulphur oxides Brake thermal efficiency Thermogravimetric Fourier transform infrared Crank angle Carbon monoxide Derivative thermogravimetric Unburned hydrocarbons Mechanical efficiency Coefficient of variation Crank angle where an accumulated heat release rate reaches 5%
performance and reduce its viscosity and corrosiveness. In the past few decades, a variety of technologies such as hydro treatment [11,12], catalytic cracking [13,14], emulsification [15e18], and steam reforming [19,20] have been proposed to upgrade biooil. However, due to some reasons, most of these technologies are unsuitable to prepare engine fuels by upgrading bio-oil [21]. For example, the applications of the bio-oil upgraded with hydro treatment and catalytic cracking were reviewed by Mortensen et al. [22]. They concluded that hydro treatment seems more promising than catalytic cracking for upgrading bio-oil economically, but challenges still obstruct the application of this technology on diesel engines. The catalyst formulation, the carbon forming mechanisms, the evaluation of high pressure, and alternative sources for hydrogen are all challenges which have to be dealt with before commercialization of the technology. Steam reforming is a specific technology for preparing hydrogen from bio-oil, thus it is not aimed at upgrading the liquid fuel of bio-oil. The applications of the emulsions of bio-oil/diesel or bio-oil/biodiesel on diesel engines were investigated by Huang et al. [16] and Prakash et al. [17] respectively. It was reported that the NOx and smoke emissions were all reduced when these emulsions were used instead of diesel fuel. However, these emulsions were unstable under the thermo conditions in the cylinder. As such, it is still very necessary to develop more suitable methods of upgrading bio-oil for transportation use. It was reported by Bayerbach et al. [23] that the low grade of biooil is partly due to the pyrolytic lignin in it, where pyrolytic lignin is produced by the pyrolysis of the lignin in biomass materials. According to this report, “solvent extraction” was proposed by Garcia et al. [24,25] and Jiang et al. [26,27] to upgrade bio-oil from another perspective. In this method, not all the constituents in bio-oil are used as a fuel. Another kind of “better fuel” is used to extract the high-quality fuel fractions and leave the pyrolytic lignin in the crude bio-oil. After the extraction, the fuel blends gradually form two layers. One layer is the residual low-grade fractions in bio-oil which mainly consists of water and some macromolecular substances such as pyrolytic lignin. Another layer is the blended liquid of the “better fuel” and the high-quality fuel fractions of bio-oil, which is called the “extraction phase liquid”. This extraction phase liquid which combines the excellent properties of the “better fuel” and the high-quality fuel fractions of bio-oil is promising to be an appropriate engine fuel. Biodiesel is the most promising renewable surrogate of mineral diesel in the world. It can be produced from various bio-resources
SOI SOC EOC BMEP BSFC LHV K HRR ITE EPBB TDC
1991
Crank angle where an accumulated heat release rate reaches 50% Crank angle where an accumulated heat release rate reaches 90% Start of injection Start of combustion End of combustion Brake mean effective pressure Brake specific fuel consumption Lower heating value Extraction efficiency Heat release rate Indicated thermal efficiency Extraction phase liquid of bio-oil/biodiesel blends Top dead center
such as unused/waste vegetable oils and animal fats by transesterification [28e30]. Since its fuel properties are quite close to those of diesel fuel, it can be successfully used in diesel engines without much adaptation [31]. It was reported that using biodiesel in diesel engines could reduce emissions of carbon monoxide (CO), unburned hydrocarbon (UHC) and soot, while NOx emission and brake specific fuel consumption (BSFC) were slightly increased [32e34]. Considering the excellent fuel properties of biodiesel, several researchers used biodiesel as the “better fuel” to upgrade bio-oil through solvent extraction. Garcia et al. [24,25] blended biodiesel and bio-oil together to extract the best fuel fractions of bio-oil by biodiesel. The biodiesel was derived from canola vegetable oil, and it was produced from the fast pyrolysis of oil mallee, pine and woody biomasses. After the extraction, the resulting fuel blends were composed of two separate phases: the lower layer rich bio-oil phase and the upper layer rich biodiesel phase. The rich biodiesel phase which is also called the “extraction phase liquid” combines the best properties of biodiesel and bio-oil. It was inferred as a promising renewable transportation fuel. Jiang and Ellis [26,27] optimized this method by adding n-octanol as a co-solvent to the blends. In their experiment, soybean-based biodiesel and softwood residue-based bio-oil were employed. After the extraction process, the upper layer extraction phase liquid was obtained. The fuel properties and thermal stability of the extraction phase liquid were tested. The results showed that the upper layer blends showed more desirable fuel properties and stable thermal properties than bio-oil for diesel engines use. Abdullah and Wu [35] also noticed the above findings and did a similar approach. In their study, the rape seed-based biodiesel was used to extract the mallee woodbased bio-oil. They indicated that the constituents extracted from bio-oil could reduce the surface tension of the extraction phase fuel. This part of fuel can be used as a renewable transportation fuel. They also found that the residue bio-oil-rich part can then be used to prepare the bioslurry fuel, which has a great application potential in Western Australia [36e38]. As such, both parts of the blends could be best used, and the whole preparation system showed high energy utilization efficiency. As reviewed above, it has been sufficiently proved that after the extraction of bio-oil by biodiesel, the upper-layer extraction phase liquid of bio-oil/biodiesel blends (denoted as EPBB) showed the great potential to be a novel renewable fuel for diesel engines. However, its practical application on diesel engines has not been tested yet. Accordingly, the main objective of this paper is to
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systematically investigate and evaluate the feasibility of using EPBB as an alternative fuel of diesel engines, in terms of fuel properties and engine performance. To this end, Thermogravimetric (TG) and Fourier transform infrared spectroscopy (FTIR) technologies will be firstly adopted to analyze the constituents transfer in the preparation of EPBB. Then the main fuel properties of EPBB under different thermal conditions are to be measured. Lastly, the combustion, economy and emission characteristics of an unmodified direct-injection diesel engine fueled with EPBB will be tested and analyzed.
2. Method and materials 2.1. Preparation of EPBB In this work, biodiesel was used to extract the soluble components from bio-oil and leave the insoluble components like pyrolytic lignin in bio-oil. Bio-oil which produced by fast pyrolysis of rice husk was provided by the University of Science and Technology of China [39]. Biodiesel produced from soybean oil via transesterification process was purchased from Xi’an Blue Sky Biological co., LTD. N-octanol which purchased from Aladdin Bio-Chem Company was added as a cosolvent which could promote the solubility of bio-oil in biodiesel. The main fuel properties of the bio-oil and biodiesel in atmospheric environment are listed in Table 1. The preparation of EPBB was mainly referred from the work conducted by Jiang and Ellis [26,27]. Since the effects of various experimental conditions on the preparation were already examined by them, this paper adopted the optimum values of their test results and mainly studied the effects of different raw materials proportions. The original fuels were blended with the stirring intensity at 1500 rpm, the blending time of 20 min, and the blending temperature at 30 C. After that, the bio-oil/biodiesel blends were kept static for 24 h. Fig. 1 presents the comparison between the blended fuels before and after the extraction. It can be observed that there are clear boundaries between the upper layer and lower layer liquid both before and after the extraction. After the extraction, the boundary retracts to a lower level and the color of the upper layer liquid becomes darker. These phenomena indicate that some constituents originally in bio-oil were extracted into biodiesel. The extraction efficiency (K) of biodiesel was then introduced to evaluate the preparation efficiency of EPBB. K was calculated by the following formula:
K¼
mup;after mup;before 100% mup;before
(1)
Where mup;before was the mass of the upper layer liquid before the extraction, which was equal to the mass of the original biodiesel. mup;after was the mass of the upper layer liquid after the extraction. mup;after mup;before was equal to the total mass of the constituents extracted from bio-oil to biodiesel. In order to determine the condition with the optimum extraction efficiency, variant conditions
Fig. 1. Comparison between the bio-oil/biodiesel blends before and after the extraction.
with different raw material proportions were tested. The initial mass ratio of bio-oil to biodiesel was set to 4:6, 5:5 and 6:4, and the mass fraction of n-octanol was varied from 1% to 7%. The experiment under each condition was repeated for three times, and the average values of three groups were used as the test result, in order to reduce the random error. Fig. 2 presents the extraction efficiencies (K) of biodiesel under different conditions with varying raw material (bio-oil, biodiesel, noctanol) proportions. When the mass ratio of bio-oil to biodiesel kept same, K first increased and then decreased as the mass fraction of n-octanol raised from 1% to 7%. When the mass fraction of noctanol was lower than 5%, more constituents were extracted from bio-oil to biodiesel with the increase of n-octanol dosage. Once the mass fraction of n-octanol exceeded 5%, phase instability happened for the bio-oil/biodiesel blends. As a result, the extraction process was obstructed and K decreased. It was also found that when the dosage of n-octanol exceeded 8%, bio-oil coking happened soon, so the result bio-oil/biodiesel blends were very unstable. When the mass fraction of n-octanol kept unchanged, K reached the highest value when the mass ratio of bio-oil to biodiesel was 5:5. The highest extraction efficiency was 21% when the mass ratio of bio-oil to biodiesel was 5:5 and the n-octanol dosage was 5%. Finally, the EPBB prepared with the highest extraction efficiency was separated from the blends for subsequent test. It can be calculated that the obtained EPBB is mainly composed of biodiesel (about 80 wt%), and
Table 1 Main fuel properties of the bio-oil and biodiesel. Properties
Density @20 C
Viscosity @20 C
Lower heating value @20 C
Water content
Carbon content
Hydrogen content
Oxygen content
Units Bio-oil Biodiesel
kg$m3 1130 873
mPa$s 11.53 6.63
MJ$kg1 15.6 38.4
Wt.% 28.3 0.0
Wt.% 37.5 78.0
Wt.% 8.3 12.0
Wt.% 53.5 10.0
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Fig. 2. Extraction efficiency (K) of biodiesel at different raw material proportions.
the percentage of the components extracted from bio-oil is low (about 20 wt%). It was also found that the EPBB prepared with the highest extraction efficiency could keep stable for more than 48 h without stratification. 2.2. TG and FTIR analyses In order to confirm the exact substances extracted from bio-oil to biodiesel, thermogravimetric (TG) analysis and Fourier transform infrared (FTIR) analysis were conducted on the crude bio-oil, biodiesel and EPBB. A TG analyzer type SDTA851 was used to record the evaporation process of the above fuels. The quality of the samples was fixed at 10 mg, and the heating rate was set to 10 C/ min. Infrared analyses were performed with a FTIR analyzer type IR Presige-21 from Shimadzu Corporation of Japan. The FTIR spectra could give a detailed description to the molecular composition of the organic fuels. 2.3. Measurement of the fuel properties The main fuel properties of biodiesel and EPBB were measured under different temperature conditions. The densities, viscosities, surface tensions and the lower heating values (LHV) of the test fuels were respectively measured by a SY-05 petroleum hydrometer, a SYD-265D-I petroleum product kinematic viscometer, a K100C fullautomatic surface tensiometer and a SXHW-2 constant-temperature digital display calorimeter, according to the Chinese National Standards GB/T 1884e2000, GB/T 10247e2008, GB/T 22237e2008 and GB/T 384e81. The measuring accuracies of above instruments were 0.5 kg,m3 , 0.01 mPa,s, 0.02 mN,m1 , 0.1 MJ, kg1, respectively. In order to reduce the random error, all the measurements were repeated for at least three times. The results were calculated from the average of three records. 2.4. Engine setup and test conditions The engine performance of biodiesel and EPBB were tested on an unmodified single-cylinder direct-injection diesel engine. The
main specifications of the engine are listed in Table 2. This is an agricultural diesel engine which is commonly used in China in the agricultural field. Since the raw materials of EPBB are produced from agricultural biomass, it is more practical to test the engine performance of EPBB in its source area on this engine. A simple schematic diagram of the engine setup which has been described previously [16] is shown in Fig. 3. The test engine was attached to an eddy current dynamometer. The air and fuel flow rates were measured by using a twisted-pair line flowmeter and a gravimetric fuel flowmeter, with the accuracy of ±1% and±0.3%, respectively. The fuel delivery timing was kept at 25 oCA BTDC, and the fuel injection signal was controlled by a mechanical device. The incylinder pressure was recorded by a Kistler piezoelectric sensor type 6125A. The output of the pressure signals was then amplified with a Kistler charge amplifier type 5015A and converted to digital signals in a Yokogawa DL750 Scope Corder. The crank angle (CA) and the injector needle lift signals were also recorded with the Yokogawa DL750 Scope Corder. The gaseous emissions (NOx, UHC, CO and CO2) were measured by HORIBA emission analyzers. In detail, the concentration of NOx was measured with a zirconia ceramic sensor type MEXA-720, the concentrations of total UHC, CO and CO2 were measured with a non-dispersive infrared analyzer type MEXA-554. The soot emission was measured with a partial flow smoke opacimeter type AVL DiSmoke 4000. The main technical characteristics of above instruments are listed in Table 3. Engine tests were carried out under different conditions concluded in Table 4. The engine was fueled with biodiesel and EPBB, and was operated on two different engine speeds (1600 rpm and 2000 rpm) under different brake mean effective pressures (BMEP). The cyclic variation is an inherent phenomenon in diesel engines which generally evaluated with the coefficient of variation (COV) [40]. In order to minimize the cyclic variation of in-cylinder pressure and get the stable value, the in-cylinder pressure was recorded for 100 sequential cycles under each condition. It was found that the COV of maximum in-cylinder pressure for all the test conditions were below 3%, which indicated an acceptable stability of the cyclic variation. The combustion and emission analyses were conducted for at least three times under each condition, such that
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Y. Huang et al. / Renewable Energy 147 (2020) 1990e2002 Table 2 Main specifications of the test engine. Model
TY1100
Type Bore stroke Displacement Piston crown shape Rated speed Rated power Compression ratio Number of nozzle holes Orifice diameter Fuel delivery static timing Fuel injection pressure
Single-cylinder, water cooled, naturally aspirated, direct-injection diesel engine 100 mm 115 mm 903 cm3 Omega 2300 rpm 11 kW 18:1 4 0.30 mm 25 oCA BTDC 20 MPa
Fig. 3. Schematic diagram of engine set up.
Table 3 Technical characteristics of the instruments for emissions measurement. Instrument
Measurement item
Range
Accuracy
Standard error (%)
Horiba MEXA-720
NOx/106 Air fuel ratio by mass Oxygen concentration/% CO/% UHC/106 CO2/% Smoke opacity extinction coefficient/m1
0e3000 9.5e200 0e25 0e10 0e10000 0e20 0e16.00
1 0.01 0.01 0.01 1 0.01 0.01
2.4 3.3 2.2 3.6 4.1 3.2 2.8
Horiba MEXA-554
SV-5Y
to reduce the random errors. The average values of these data were taken as the test results. The standard errors of most measured parameters are listed in Table 3, and the uncertainties of all the data were within 95% confidence level.
Table 4 Engine test conditions. Engine speed/rpm Brake mean effective pressures/MPa Tested fuels
1600, 2000 0.14, 0.28, 0.42, 0.56, 0.70 Biodiesel, EPBB
3. Results and discussion 3.1. TG and FTIR analyses results 3.1.1. TG analysis result The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of bio-oil, biodiesel and EPBB are shown in Fig. 4. TG and DTG curves indicated that the initial evaporation timing and maximum weight loss timing of EPBB were earlier than those of biodiesel. This may be due to that the light components which evaporate under lower temperature were extracted from bio-oil to biodiesel. As a result, the evaporation process of biodiesel was
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shown in Fig. 6. Although the FTIR spectra of EPBB were similar to those of biodiesel, there were still some difference between their absorption peaks due to the existence of the components extracted from bio-oil. In detail, there were extra absorption peaks in the FTIR spectra of EPBB at 3462 cm1, 1595 cm1 and 1514 cm1. Moreover, the absorption peaks at 1463 cm1, 1361 cm1, 1246 cm1, 1170 cm1, 1118 cm1 and 1016 cm1 were strengthened. By referring these changed absorption peaks to those of bio-oil in Table 5, the exact components extracted from bio-oil can be finally identified. The results show that alcohols, ethers, ketones and carboxylic acids were extracted from bio-oil to biodiesel in the extraction process.
3.2. Main fuel properties
Fig. 4. TG and DTG curves of bio-oil, biodiesel and EPBB.
accelerated. In addition, after the TG curves reached their minimum, the residual weights of the test fuels were different. The residual weight of bio-oil was the largest. This is mainly because that the macromolecular substances in bio-oil like pyrolytic lignin were hard to evaporate, and the constituents in bio-oil were possible to polymerize in the heating process. The residual weight of EPBB was equal to that of biodiesel and lighter than that of biooil. This indicated that most of the constituents extracted from biooil to biodiesel were easy to evaporate. 3.1.2. FTIR analysis result FTIR spectra analyses were conducted on bio-oil, biodiesel and EPBB respectively, in order to identify the exact components that extracted from bio-oil to biodiesel. Firstly, the FTIR spectra of bio-oil were tested and shown in Fig. 5. According to the locations of various absorption peaks, different radicals in bio-oil were identified. Combining the identification of variant radicals, different components in bio-oil were then identified as shown in Table 5 [41]. There were water, alcohols, ethers, ketones, carboxylic acids and furfurals in bio-oil. Secondly, the FTIR spectra of biodiesel and EPBB were tested and
3.2.1. Densities Fig. 7 shows the densities of biodiesel and EPBB at different temperatures. The densities of biodiesel and EPBB showed similar variation trend with the temperature. They decreased linearly with the increase of temperature at a same rate. This was because that EPBB was mainly consisted of biodiesel (about 80 wt%). At the same temperature, the density of EPBB was slightly higher than that of biodiesel. This indicated that not only the light constituents such as alcohols, ethers, but also carboxylic acids and some macromolecular compounds with higher densities as compared with biodiesel were also extracted from bio-oil to biodiesel.
3.2.2. Viscosities Fig. 8 shows the variations of the viscosities of biodiesel and EPBB with temperature. The viscosities of biodiesel and EPBB decreased with the increase of temperature at a same rate. When the temperature remained unchanged, the viscosity of EPBB was lower than that of biodiesel. This was due to that most of the constituents extracted from bio-oil to biodiesel have lower viscosities than biodiesel.
3.2.3. Surface tensions Fig. 9 presents the surface tensions of biodiesel and EPBB at different temperatures. The surface tensions of biodiesel and EPBB decreased at a same rate as the temperature rose. At the same temperature, the surface tension of EPBB was slightly lower than that of biodiesel. This was also due to that the surface tensions of the constituents extracted from bio-oil were lower than that of biodiesel.
3.2.4. Lower heating values Table 6 shows the lower heating values (LHV) of bio-oil, biodiesel and EPBB. Although the LHV of bio-oil was much lower than that of biodiesel, the LHV of EPBB was slightly lower than that of biodiesel. The reason was that a higher percentage of the constituents extracted from bio-oils have smaller lower heating value, while EPBB was mainly consisted of biodiesel.
3.3. Engine performance
Fig. 5. FTIR spectra of bio-oil.
3.3.1. Combustion characteristics The combustion characteristics were analyzed in terms of the in-cylinder pressure and temperature, heat release rate (HRR), ignition delay, premixed combustion and total combustion duration. HRR was obtained by calculating the following equation [16]:
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Table 5 Identification of the components in bio-oil by FTIR spectra. Components
Water
Alcohols
Radicals Absorption peaks
OeH 3200e3600
OeH 3200e3600
CeO 1050, 1080, 1265
Ethers
Ketones
CeO 1060e1150
C¼O 1715
Carboxylic Acids CeCeC 1100e1300
CeO 1395e1440
OeH 1210e1320
Carboxyl 1300-1420, 1550-1610-
Fig. 8. Variations of the viscosities of biodiesel and EPBB with temperature.
Fig. 6. FTIR spectra of biodiesel and EPBB.
Fig. 9. Variations of the surface tensions of biodiesel and EPBB with temperature.
Fig. 7. Variations of the densities of biodiesel and EPBB with temperature.
dp W p dV þ dQ þ R1cv V d4 þ p dV d4 d4 d4 dQB ¼ d4 1 l u cv T m vu 1 Hu vl mB dQW ¼ hc AðT TwÞ d4
Table 6 Lower heating values of bio-oil, biodiesel and EPBB. Fuel type Lower heating value/MJ$kg
1
Bio-oil
Biodiesel
EPBB
15.6
39.8
38.3
(2)
(3)
Where p, V and T are the pressure, volume and temperature of the in-cylinder charge. The heat transfer rate is given by Equation (3) and the heat transfer coefficient hc is determined from the
Woschni correlation. A and Tw are the area and average temperature of the heat transfer surface. Hu is the LHV of the fuel, relative air-to-fuel ratio l is the ratio of the actual air-to-fuel ratio to the stoichiometric ratio. m and mB are the mass of in-cylinder charge and the injected fuel. The definitions of the different phases for combustion analysis were mainly according to the methodology presented by Nour et al. [42]. Ignition delay was defined as the crank angle interval from the
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Fig. 10. In-cylinder pressure and heat release rate for biodiesel and EPBB. Fig. 11. In-cylinder temperature for biodiesel and EPBB.
start of fuel injection (SOI) to the start of combustion (SOC). The total combustion duration was the crank angle interval from SOC to the end of combustion (EOC). In detail, SOI was identified as the crank angle when the injector needle reached the 10% of its maximum lift. SOC and EOC were respectively identified as CA05 and CA90. Premixed combustion was identified as the crank angle interval from CA05 to CA50. CA05, CA50 and CA90 were the crank angles when the accumulated heat release rate was 5%, 50% and 90% of the total heat release rate, respectively. Fig. 10 and Fig. 11 shows the in-cylinder pressures and HRRs as well as in-cylinder temperature of the engine fueled with biodiesel and EPBB under the condition with BMEP at 0.42 MPa, and the engine speed at 1600 rpm and 2000 rpm respectively. In order to concentrate on the major combustion process, only a part of the data history was selected and shown. The curves start from fuel static delivery timing (25 oCA BTDC) to near EOC (40 oCA ATDC). Under both engine speed conditions, although the in-cylinder pressure and temperature rose later, they rose faster when EPBB was used instead of biodiesel. The curve tails of the in-cylinder pressure and temperature of EPBB finally become overlapped with those of biodiesel, which indicated a similar EOC for the two test fuels. In addition, the peak values of the in-cylinder pressure and temperature of EPBB were slightly higher than those of biodiesel at the engine speed of 1600 rpm, while they were slightly lower than those of biodiesel at 2000 rpm. The peak values of HRR
of EPBB were obviously higher than those of biodiesel under two engine speed conditions. It can also be inferred from the faster growth of in-cylinder pressure in Fig. 10 and the positive correlation between and the peak values of HRR and the pressure rise rate that the peak pressure rise rates of EPBB were also evidently higher than those of biodiesel [16]. Fig. 12 presents the ignition delay of biodiesel and EPBB under different BMEP conditions at the engine speed of 1600 rpm and 2000 rpm respectively. The ignition delay of EPBB was longer than that of biodiesel under all the conditions. The ignition delay consists of a physical delay and a chemical delay [42]. Generally, the physical delay is mainly influenced by the fuel density, viscosity and volatility, and the chemical delay is dependent on the fuel cetane number [43]. On the one hand, EPBB has the lower viscosity and surface tension (see Figs. 8 and 9) as well as the higher volatility (see Fig. 4) when compared with biodiesel. Therefore, the atomization and evaporation performance of EPBB was better. This better atomization and evaporation characteristic contraposed the effect of its higher vaporization enthalpy on the physical delay, and the proportion of physical delay was thereby minimized. On the other hand, the cetane number of EPBB was expected to be lower than that of biodiesel, due to the addition of the components with low cetane number in EPBB, such as alcohols, ketones etc. As a result, the chemical delay of EPBB was prolonged [43]. Overall, the
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chemical delay played a dominant role in the ignition delay, as such EPBB revealed a longer ignition delay as compared with biodiesel. The durations of premixed combustion and total combustion for biodiesel and EPBB under different BMEP conditions at 1600 rpm are shown in Fig. 13. As can be observed, the premixed combustion duration of EPBB were longer than that of biodiesel under all the conditions. This was mainly attributed to two factors. First, with the longer ignition delay of EPBB, more fuel was injected to the cylinder before the ignition. Second, the atomization and evaporation performance of EPBB was better as stated above. Under the coupling effects of these two factors, much air/fuel mixture was prepared for the premixed combustion. The premixed combustion duration of EPBB was thereby prolonged. Moreover, the total combustion duration of EPBB was however shorter than that of biodiesel under all the conditions. This was also mainly due to two reasons. On the one hand, the fuel atomization and evaporation of EPBB were better than those of biodiesel, so that the fuel/air mixing was promoted
Fig. 12. Ignition delays of biodiesel and EPBB. Fig. 14. BTEs of the engine fueled with biodiesel and EPBB.
Fig. 13. Premixed combustion duration and total combustion duration of biodiesel and EPBB.
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during the combustion. On the other hand, the oxygen content of EPBB was higher than that of biodiesel. The diffusion combustion of EPBB was improved as a result of the increase of fuel-bound oxygen at crucial zones. Hence, the whole combustion process was optimized and the total combustion duration was shortened. With a shorter total combustion duration, the EOC of EPBB was thereby about same to that of biodiesel despite of a longer ignition duration (see Fig. 11).
3.3.2. Fuel economy Fig. 14 shows the brake thermal efficiencies (BTE) of biodiesel and EPBB under different BMEP conditions at the engine speed of 1600 rpm. The BTE of EPBB was equivalent to that of biodiesel under all the conditions. BTE mainly depends on the indicated thermal efficiencies (ITE) and the mechanical efficiency, as described in the following formula:
het ¼ hit hm
(4)
Where het is the BTE, hit is the ITE, hm is the mechanical efficiency. As can be seen in the HRR curves in Fig. 10, the peak value of HRR was closer to the top dead center (TDC) when EPBB was used instead of biodiesel. This indicated that the combustion heat release of EPBB was more concentrated near the TDC, thus the ITE of EPBB were higher [44]. However, as concluded above in Fig. 10, EPBB has the higher peak values of the in-cylinder pressure and pressure rise rate, which represented a rougher combustion process and a lower mechanical efficiency. Under the coupling effects of these two factors, the BTEs of the two fuels showed minor difference. Fig. 15 shows the brake specific fuel consumptions (BSFC) of biodiesel and EPBB under different BMEP conditions at the engine speed of 1600 rpm. The BSFC of EPBB was slightly higher than that of biodiesel. BSFC is calculated from
be ¼
3:6 106 100% Hu het
(5)
where be is BSFC, Hu is the LHV of the fuel, het is the BTE. The BTEs of the two fuels were equivalent, while the LHV of EPBB was slightly lower than that of biodiesel as presented in Table 6. Thus, the BSFC of EPBB was higher than biodiesel.
Fig. 15. BSFCs of the engine fueled with biodiesel and EPBB.
Fig. 16. NOx emissions of biodiesel and EPBB under different loads.
3.3.3. Emission characteristics Fig. 16 presents the NOx emissions of biodiesel and EPBB under various loads at the engine speed of 1600 rpm and 2000 rpm. The NOx emission of EPBB was lower than that of biodiesel under all the conditions. It is well known that the formation of NOx in diesel engines is mainly according to the Zeldovich mechanism. The local flame temperature, high-temperature duration and the oxygen concentration are the main factors deciding the emission of NOx [45,46]. Since most of the constituents extracted from bio-oil have lower calorific value and higher enthalpy of evaporation when compared with biodiesel, the local flame temperature of EPBB was thereby lower than biodiesel. In addition, as can be seen from Fig. 11, the high temperature duration of EPBB was shorter than that of biodiesel. As such, EPBB revealed a lower NOx emissions despite of its higher oxygen content. The soot emissions of biodiesel and EPBB were characterized by the smoke opacity, as shown in Fig. 17. In comparison with biodiesel, the soot emission of EPBB was lower for most BMEP conditions. The emission of soot mainly depends on its formation and oxidation processes, where the latter process plays a major role. In compression ignition engines, soot is mainly formed by the partial oxidation and pyrolysis of the evaporate fuel under hightemperature and anoxic environment [47]. The atomization and evaporation performance of EPBB was better than that of biodiesel as stated before, so the anoxic region of over-rich mixture reduces and less soot was formed as a result. On the other hand, the addition of oxygen-enriched constituents from bio-oil increased the
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Y. Huang et al. / Renewable Energy 147 (2020) 1990e2002
Fig. 17. Soot emissions of biodiesel and EPBB under different loads.
oxygen content of EPBB, as such soot oxidation in the flame zone was promoted for EPBB [48]. Fig. 18 and Fig. 19 present the CO and UHC emissions of biodiesel and EPBB. The CO and UHC emissions of EPBB were respectively lower than those of biodiesel under all the conditions. The lower CO emission of EPBB was a predictable result, since the higher oxygen content of EPBB could reduce the region with over-rich mixture and suppress the incomplete combustion. On the other hand, the lower UHC emissions of EPBB was a surprising but welcome result. Generally, the longer ignition delay of EPBB leads to an increase of the over-lean region of air/fuel mixture, and thereby increases the UHC emissions. However, it can be inferred from the test results that the ignition delay may not have the leading role in UHC emissions of EPBB. The lower UHC emission of EPBB were likely attribute to the improvement of its atomization and evaporation performance. With this improvement, the regions with over-rich mixture were reduced, and the UHC emissions of EPBB were decreased despite of the increase of the over-lean region. 4. Conclusions In this paper, a pure biomass-based novel fuel, namely the extraction phase liquid of bio-oil/biodiesel blends (EPBB) was firstly
Fig. 18. CO emissions of biodiesel and EPBB under different loads.
prepared. In order to investigate its usability as an alternative engine fuel, the preparation efficiency, molecular compositions, thermophysical properties and diesel engine performance of EPBB were successively tested. Based on the test results, following conclusions are summarized. 1. When the initial mass ratio of biodiesel to bio-oil is 5:5, 5% noctanol was added as a co-solvent, the extraction efficiency K of biodiesel reaches the maximum. The highest extraction efficiency is about 21%, so that EPBB consists of biodiesel (about 80 wt%) and the best fractions in bio-oil (about 20 wt%). The whole preparation process is very convenient, and the thermal stability of EPBB is high enough for it to be used as an engine fuel. 2. During the extraction process, alcohols, ethers, ketones, carboxylic acids and some macromolecular compounds are extracted from bio-oil to biodiesel. As a result, EPBB shows a higher density, lower viscosity, surface tension and LHV when compared with biodiesel. The atomization and evaporation performance of EPBB is thereby better than those of biodiesel.
Y. Huang et al. / Renewable Energy 147 (2020) 1990e2002
[2]
[3]
[4] [5] [6] [7]
[8]
[9] [10] [11] [12]
[13] [14]
[15]
[16]
[17]
[18] Fig. 19. UHC emissions of biodiesel and EPBB under different loads.
[19] [20]
3. In comparison with biodiesel, EPBB shows a longer ignition delay. Although the premixed combustion duration of EPBB is longer, its diffusion combustion and total combustion durations are shorter. The fuel economy of EPBB is comparable to that of biodiesel. More importantly, NOx, soot, CO and UHC emissions of EPBB are all more or less decreased when compared to biodiesel. 4. On the one hand, the poor fuel properties of the crude bio-oil obstruct its direct application as an engine fuel. On the other hand, the fuel properties and engine performance of EPBB are close or even better than those of biodiesel. Considering these two facts, it seems that preparing EPBB through the extraction of bio-oil by biodiesel will be a better choice to apply both bio-oil and biodiesel in diesel engines.
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[23]
[24]
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[26]
Acknowledgements
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The authors wish to express their deep thanks to the National Key Research and Development Program of China (No. 2018YFB0105900) for their support of the project.
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Investigation on fuel properties and engine performance of the extraction phase liquid of bio-oil/biodiesel blends Yongcheng Huang*, Yaoting Li, Xudong Han, Jiating Zhang, Kun Luo, Shangsheng Yang, Jiyuan Wang School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, 710049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 May 2019 Received in revised form 26 September 2019 Accepted 6 October 2019 Available online 7 October 2019
In this paper, biodiesel was used to upgrade bio-oil by extracting the high-quality fuel fractions in bio-oil through solvent extraction. After the extraction, the upper layer blends of biodiesel and the high-quality bio-oil components, also called the “extraction phase liquid”, was obtained and denoted as EPBB. Firstly, Thermogravimetric (TG) and Fourier transform infrared (FTIR) analyses indicated that the light components originally in bio-oil, such as alcohols, ethers, ketones and carboxylic acids were extracted from bio-oil to biodiesel and formed the EPBB. Then the fuel properties of EPBB were measured at different temperatures. The results showed that the density of EPBB was higher, while its viscosity, surface tension and lower heating value (LHV) were lower than those of biodiesel. Accordingly, EPBB revealed better atomization and evaporation characteristics as compared to biodiesel. Finally, the engine performance of EPBB was tested on an unmodified direct-injection diesel engine. When compared with biodiesel, the ignition delay of EPBB was longer, the premixed combustion duration was longer, while the total combustion duration was shorter. The fuel economy of EPBB was comparable to that of biodiesel, and the NOx, soot, CO and UHC emissions of EPBB were all lower than those of biodiesel. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Bio-oil Biodiesel Fuel property Diesel engine Combustion Emissions
1. Introduction With the gradual depletion of petroleum reserves and the widespread concerns about the environmental issues such as air pollution and greenhouse effect, there is a huge demand for vehicle engines, especially diesel engines to develop the clean and renewable alternative fuels. Liquid biofuels, which mainly refer to bioalcohols, biodiesels and bio-oils are regarded as promising alternative fuels for petroleum diesel. The superiority of biofuels is mainly attributed to their renewability and low emissions, especially in reducing the greenhouse gases (GHG) and defeating the smoke-NOx trade-off of fossil diesel [1e3]. Bio-oil, also known as biomass pyrolysis oil, is a dark brown, free flowing liquid fuel. It can be produced through the fast pyrolysis of a variety of renewable biomass resources such as waste wood, corn straw and rice husk [4]. Typically, it has high proportions of oxygen, water, acid and solids, high viscosity and surface tension, low heating value and unstable thermal properties [5]. Therefore, it is a
* Corresponding author. E-mail address: [email protected] (Y. Huang). https://doi.org/10.1016/j.renene.2019.10.028 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
kind of lower-grade fuel when compared with fossil fuels. However, bio-oil has several distinct advantages over fossil fuels on energy and environmental aspects. It has negligible contents of sulphur, nitrogen and ash, so it produces the lower Nitrogen Oxides (NOx) and Sulphur Oxides (SOx) emissions under most conditions when compared with conventional fossil diesel [6]. In addition, bio-oil is environmental-friendly since it is carbon neutral and biodegradable. Consequently, bio-oil has been recognized as a potential alternative fuel for transportation vehicles, and a large number of researches have been conducted on the application of bio-oil on diesel engines. Regrettably, bio-oil cannot ignite spontaneously without adding additives, and injector nozzle coking and clogging are also observed soon after the combustion of bio-oil [7]. Based on these facts, various methods such as adding additives [8], hot filtration [9] and diesel ignition improvers [10] were later proposed to promote the ignition and combustion of bio-oil in diesel engines. However, due to the poor properties of the crude bio-oil, the problems of fuel delivery system corrosion and injector nozzle clogging were still not solved by these methods. Therefore, in order to utilize bio-oil in diesel engines without much engine modification, an upgrade treatment of the crude bio-oil is necessary to improve its ignition
Y. Huang et al. / Renewable Energy 147 (2020) 1990e2002
Nomenclature
CA50
NOx SOx BTE TG FTIR CA CO DTG UHC
CA90
hm COV CA05
Nitrogen oxides Sulphur oxides Brake thermal efficiency Thermogravimetric Fourier transform infrared Crank angle Carbon monoxide Derivative thermogravimetric Unburned hydrocarbons Mechanical efficiency Coefficient of variation Crank angle where an accumulated heat release rate reaches 5%
performance and reduce its viscosity and corrosiveness. In the past few decades, a variety of technologies such as hydro treatment [11,12], catalytic cracking [13,14], emulsification [15e18], and steam reforming [19,20] have been proposed to upgrade biooil. However, due to some reasons, most of these technologies are unsuitable to prepare engine fuels by upgrading bio-oil [21]. For example, the applications of the bio-oil upgraded with hydro treatment and catalytic cracking were reviewed by Mortensen et al. [22]. They concluded that hydro treatment seems more promising than catalytic cracking for upgrading bio-oil economically, but challenges still obstruct the application of this technology on diesel engines. The catalyst formulation, the carbon forming mechanisms, the evaluation of high pressure, and alternative sources for hydrogen are all challenges which have to be dealt with before commercialization of the technology. Steam reforming is a specific technology for preparing hydrogen from bio-oil, thus it is not aimed at upgrading the liquid fuel of bio-oil. The applications of the emulsions of bio-oil/diesel or bio-oil/biodiesel on diesel engines were investigated by Huang et al. [16] and Prakash et al. [17] respectively. It was reported that the NOx and smoke emissions were all reduced when these emulsions were used instead of diesel fuel. However, these emulsions were unstable under the thermo conditions in the cylinder. As such, it is still very necessary to develop more suitable methods of upgrading bio-oil for transportation use. It was reported by Bayerbach et al. [23] that the low grade of biooil is partly due to the pyrolytic lignin in it, where pyrolytic lignin is produced by the pyrolysis of the lignin in biomass materials. According to this report, “solvent extraction” was proposed by Garcia et al. [24,25] and Jiang et al. [26,27] to upgrade bio-oil from another perspective. In this method, not all the constituents in bio-oil are used as a fuel. Another kind of “better fuel” is used to extract the high-quality fuel fractions and leave the pyrolytic lignin in the crude bio-oil. After the extraction, the fuel blends gradually form two layers. One layer is the residual low-grade fractions in bio-oil which mainly consists of water and some macromolecular substances such as pyrolytic lignin. Another layer is the blended liquid of the “better fuel” and the high-quality fuel fractions of bio-oil, which is called the “extraction phase liquid”. This extraction phase liquid which combines the excellent properties of the “better fuel” and the high-quality fuel fractions of bio-oil is promising to be an appropriate engine fuel. Biodiesel is the most promising renewable surrogate of mineral diesel in the world. It can be produced from various bio-resources
SOI SOC EOC BMEP BSFC LHV K HRR ITE EPBB TDC
1991
Crank angle where an accumulated heat release rate reaches 50% Crank angle where an accumulated heat release rate reaches 90% Start of injection Start of combustion End of combustion Brake mean effective pressure Brake specific fuel consumption Lower heating value Extraction efficiency Heat release rate Indicated thermal efficiency Extraction phase liquid of bio-oil/biodiesel blends Top dead center
such as unused/waste vegetable oils and animal fats by transesterification [28e30]. Since its fuel properties are quite close to those of diesel fuel, it can be successfully used in diesel engines without much adaptation [31]. It was reported that using biodiesel in diesel engines could reduce emissions of carbon monoxide (CO), unburned hydrocarbon (UHC) and soot, while NOx emission and brake specific fuel consumption (BSFC) were slightly increased [32e34]. Considering the excellent fuel properties of biodiesel, several researchers used biodiesel as the “better fuel” to upgrade bio-oil through solvent extraction. Garcia et al. [24,25] blended biodiesel and bio-oil together to extract the best fuel fractions of bio-oil by biodiesel. The biodiesel was derived from canola vegetable oil, and it was produced from the fast pyrolysis of oil mallee, pine and woody biomasses. After the extraction, the resulting fuel blends were composed of two separate phases: the lower layer rich bio-oil phase and the upper layer rich biodiesel phase. The rich biodiesel phase which is also called the “extraction phase liquid” combines the best properties of biodiesel and bio-oil. It was inferred as a promising renewable transportation fuel. Jiang and Ellis [26,27] optimized this method by adding n-octanol as a co-solvent to the blends. In their experiment, soybean-based biodiesel and softwood residue-based bio-oil were employed. After the extraction process, the upper layer extraction phase liquid was obtained. The fuel properties and thermal stability of the extraction phase liquid were tested. The results showed that the upper layer blends showed more desirable fuel properties and stable thermal properties than bio-oil for diesel engines use. Abdullah and Wu [35] also noticed the above findings and did a similar approach. In their study, the rape seed-based biodiesel was used to extract the mallee woodbased bio-oil. They indicated that the constituents extracted from bio-oil could reduce the surface tension of the extraction phase fuel. This part of fuel can be used as a renewable transportation fuel. They also found that the residue bio-oil-rich part can then be used to prepare the bioslurry fuel, which has a great application potential in Western Australia [36e38]. As such, both parts of the blends could be best used, and the whole preparation system showed high energy utilization efficiency. As reviewed above, it has been sufficiently proved that after the extraction of bio-oil by biodiesel, the upper-layer extraction phase liquid of bio-oil/biodiesel blends (denoted as EPBB) showed the great potential to be a novel renewable fuel for diesel engines. However, its practical application on diesel engines has not been tested yet. Accordingly, the main objective of this paper is to
1992
Y. Huang et al. / Renewable Energy 147 (2020) 1990e2002
systematically investigate and evaluate the feasibility of using EPBB as an alternative fuel of diesel engines, in terms of fuel properties and engine performance. To this end, Thermogravimetric (TG) and Fourier transform infrared spectroscopy (FTIR) technologies will be firstly adopted to analyze the constituents transfer in the preparation of EPBB. Then the main fuel properties of EPBB under different thermal conditions are to be measured. Lastly, the combustion, economy and emission characteristics of an unmodified direct-injection diesel engine fueled with EPBB will be tested and analyzed.
2. Method and materials 2.1. Preparation of EPBB In this work, biodiesel was used to extract the soluble components from bio-oil and leave the insoluble components like pyrolytic lignin in bio-oil. Bio-oil which produced by fast pyrolysis of rice husk was provided by the University of Science and Technology of China [39]. Biodiesel produced from soybean oil via transesterification process was purchased from Xi’an Blue Sky Biological co., LTD. N-octanol which purchased from Aladdin Bio-Chem Company was added as a cosolvent which could promote the solubility of bio-oil in biodiesel. The main fuel properties of the bio-oil and biodiesel in atmospheric environment are listed in Table 1. The preparation of EPBB was mainly referred from the work conducted by Jiang and Ellis [26,27]. Since the effects of various experimental conditions on the preparation were already examined by them, this paper adopted the optimum values of their test results and mainly studied the effects of different raw materials proportions. The original fuels were blended with the stirring intensity at 1500 rpm, the blending time of 20 min, and the blending temperature at 30 C. After that, the bio-oil/biodiesel blends were kept static for 24 h. Fig. 1 presents the comparison between the blended fuels before and after the extraction. It can be observed that there are clear boundaries between the upper layer and lower layer liquid both before and after the extraction. After the extraction, the boundary retracts to a lower level and the color of the upper layer liquid becomes darker. These phenomena indicate that some constituents originally in bio-oil were extracted into biodiesel. The extraction efficiency (K) of biodiesel was then introduced to evaluate the preparation efficiency of EPBB. K was calculated by the following formula:
K¼
mup;after mup;before 100% mup;before
(1)
Where mup;before was the mass of the upper layer liquid before the extraction, which was equal to the mass of the original biodiesel. mup;after was the mass of the upper layer liquid after the extraction. mup;after mup;before was equal to the total mass of the constituents extracted from bio-oil to biodiesel. In order to determine the condition with the optimum extraction efficiency, variant conditions
Fig. 1. Comparison between the bio-oil/biodiesel blends before and after the extraction.
with different raw material proportions were tested. The initial mass ratio of bio-oil to biodiesel was set to 4:6, 5:5 and 6:4, and the mass fraction of n-octanol was varied from 1% to 7%. The experiment under each condition was repeated for three times, and the average values of three groups were used as the test result, in order to reduce the random error. Fig. 2 presents the extraction efficiencies (K) of biodiesel under different conditions with varying raw material (bio-oil, biodiesel, noctanol) proportions. When the mass ratio of bio-oil to biodiesel kept same, K first increased and then decreased as the mass fraction of n-octanol raised from 1% to 7%. When the mass fraction of noctanol was lower than 5%, more constituents were extracted from bio-oil to biodiesel with the increase of n-octanol dosage. Once the mass fraction of n-octanol exceeded 5%, phase instability happened for the bio-oil/biodiesel blends. As a result, the extraction process was obstructed and K decreased. It was also found that when the dosage of n-octanol exceeded 8%, bio-oil coking happened soon, so the result bio-oil/biodiesel blends were very unstable. When the mass fraction of n-octanol kept unchanged, K reached the highest value when the mass ratio of bio-oil to biodiesel was 5:5. The highest extraction efficiency was 21% when the mass ratio of bio-oil to biodiesel was 5:5 and the n-octanol dosage was 5%. Finally, the EPBB prepared with the highest extraction efficiency was separated from the blends for subsequent test. It can be calculated that the obtained EPBB is mainly composed of biodiesel (about 80 wt%), and
Table 1 Main fuel properties of the bio-oil and biodiesel. Properties
Density @20 C
Viscosity @20 C
Lower heating value @20 C
Water content
Carbon content
Hydrogen content
Oxygen content
Units Bio-oil Biodiesel
kg$m3 1130 873
mPa$s 11.53 6.63
MJ$kg1 15.6 38.4
Wt.% 28.3 0.0
Wt.% 37.5 78.0
Wt.% 8.3 12.0
Wt.% 53.5 10.0
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Fig. 2. Extraction efficiency (K) of biodiesel at different raw material proportions.
the percentage of the components extracted from bio-oil is low (about 20 wt%). It was also found that the EPBB prepared with the highest extraction efficiency could keep stable for more than 48 h without stratification. 2.2. TG and FTIR analyses In order to confirm the exact substances extracted from bio-oil to biodiesel, thermogravimetric (TG) analysis and Fourier transform infrared (FTIR) analysis were conducted on the crude bio-oil, biodiesel and EPBB. A TG analyzer type SDTA851 was used to record the evaporation process of the above fuels. The quality of the samples was fixed at 10 mg, and the heating rate was set to 10 C/ min. Infrared analyses were performed with a FTIR analyzer type IR Presige-21 from Shimadzu Corporation of Japan. The FTIR spectra could give a detailed description to the molecular composition of the organic fuels. 2.3. Measurement of the fuel properties The main fuel properties of biodiesel and EPBB were measured under different temperature conditions. The densities, viscosities, surface tensions and the lower heating values (LHV) of the test fuels were respectively measured by a SY-05 petroleum hydrometer, a SYD-265D-I petroleum product kinematic viscometer, a K100C fullautomatic surface tensiometer and a SXHW-2 constant-temperature digital display calorimeter, according to the Chinese National Standards GB/T 1884e2000, GB/T 10247e2008, GB/T 22237e2008 and GB/T 384e81. The measuring accuracies of above instruments were 0.5 kg,m3 , 0.01 mPa,s, 0.02 mN,m1 , 0.1 MJ, kg1, respectively. In order to reduce the random error, all the measurements were repeated for at least three times. The results were calculated from the average of three records. 2.4. Engine setup and test conditions The engine performance of biodiesel and EPBB were tested on an unmodified single-cylinder direct-injection diesel engine. The
main specifications of the engine are listed in Table 2. This is an agricultural diesel engine which is commonly used in China in the agricultural field. Since the raw materials of EPBB are produced from agricultural biomass, it is more practical to test the engine performance of EPBB in its source area on this engine. A simple schematic diagram of the engine setup which has been described previously [16] is shown in Fig. 3. The test engine was attached to an eddy current dynamometer. The air and fuel flow rates were measured by using a twisted-pair line flowmeter and a gravimetric fuel flowmeter, with the accuracy of ±1% and±0.3%, respectively. The fuel delivery timing was kept at 25 oCA BTDC, and the fuel injection signal was controlled by a mechanical device. The incylinder pressure was recorded by a Kistler piezoelectric sensor type 6125A. The output of the pressure signals was then amplified with a Kistler charge amplifier type 5015A and converted to digital signals in a Yokogawa DL750 Scope Corder. The crank angle (CA) and the injector needle lift signals were also recorded with the Yokogawa DL750 Scope Corder. The gaseous emissions (NOx, UHC, CO and CO2) were measured by HORIBA emission analyzers. In detail, the concentration of NOx was measured with a zirconia ceramic sensor type MEXA-720, the concentrations of total UHC, CO and CO2 were measured with a non-dispersive infrared analyzer type MEXA-554. The soot emission was measured with a partial flow smoke opacimeter type AVL DiSmoke 4000. The main technical characteristics of above instruments are listed in Table 3. Engine tests were carried out under different conditions concluded in Table 4. The engine was fueled with biodiesel and EPBB, and was operated on two different engine speeds (1600 rpm and 2000 rpm) under different brake mean effective pressures (BMEP). The cyclic variation is an inherent phenomenon in diesel engines which generally evaluated with the coefficient of variation (COV) [40]. In order to minimize the cyclic variation of in-cylinder pressure and get the stable value, the in-cylinder pressure was recorded for 100 sequential cycles under each condition. It was found that the COV of maximum in-cylinder pressure for all the test conditions were below 3%, which indicated an acceptable stability of the cyclic variation. The combustion and emission analyses were conducted for at least three times under each condition, such that
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Y. Huang et al. / Renewable Energy 147 (2020) 1990e2002 Table 2 Main specifications of the test engine. Model
TY1100
Type Bore stroke Displacement Piston crown shape Rated speed Rated power Compression ratio Number of nozzle holes Orifice diameter Fuel delivery static timing Fuel injection pressure
Single-cylinder, water cooled, naturally aspirated, direct-injection diesel engine 100 mm 115 mm 903 cm3 Omega 2300 rpm 11 kW 18:1 4 0.30 mm 25 oCA BTDC 20 MPa
Fig. 3. Schematic diagram of engine set up.
Table 3 Technical characteristics of the instruments for emissions measurement. Instrument
Measurement item
Range
Accuracy
Standard error (%)
Horiba MEXA-720
NOx/106 Air fuel ratio by mass Oxygen concentration/% CO/% UHC/106 CO2/% Smoke opacity extinction coefficient/m1
0e3000 9.5e200 0e25 0e10 0e10000 0e20 0e16.00
1 0.01 0.01 0.01 1 0.01 0.01
2.4 3.3 2.2 3.6 4.1 3.2 2.8
Horiba MEXA-554
SV-5Y
to reduce the random errors. The average values of these data were taken as the test results. The standard errors of most measured parameters are listed in Table 3, and the uncertainties of all the data were within 95% confidence level.
Table 4 Engine test conditions. Engine speed/rpm Brake mean effective pressures/MPa Tested fuels
1600, 2000 0.14, 0.28, 0.42, 0.56, 0.70 Biodiesel, EPBB
3. Results and discussion 3.1. TG and FTIR analyses results 3.1.1. TG analysis result The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of bio-oil, biodiesel and EPBB are shown in Fig. 4. TG and DTG curves indicated that the initial evaporation timing and maximum weight loss timing of EPBB were earlier than those of biodiesel. This may be due to that the light components which evaporate under lower temperature were extracted from bio-oil to biodiesel. As a result, the evaporation process of biodiesel was
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shown in Fig. 6. Although the FTIR spectra of EPBB were similar to those of biodiesel, there were still some difference between their absorption peaks due to the existence of the components extracted from bio-oil. In detail, there were extra absorption peaks in the FTIR spectra of EPBB at 3462 cm1, 1595 cm1 and 1514 cm1. Moreover, the absorption peaks at 1463 cm1, 1361 cm1, 1246 cm1, 1170 cm1, 1118 cm1 and 1016 cm1 were strengthened. By referring these changed absorption peaks to those of bio-oil in Table 5, the exact components extracted from bio-oil can be finally identified. The results show that alcohols, ethers, ketones and carboxylic acids were extracted from bio-oil to biodiesel in the extraction process.
3.2. Main fuel properties
Fig. 4. TG and DTG curves of bio-oil, biodiesel and EPBB.
accelerated. In addition, after the TG curves reached their minimum, the residual weights of the test fuels were different. The residual weight of bio-oil was the largest. This is mainly because that the macromolecular substances in bio-oil like pyrolytic lignin were hard to evaporate, and the constituents in bio-oil were possible to polymerize in the heating process. The residual weight of EPBB was equal to that of biodiesel and lighter than that of biooil. This indicated that most of the constituents extracted from biooil to biodiesel were easy to evaporate. 3.1.2. FTIR analysis result FTIR spectra analyses were conducted on bio-oil, biodiesel and EPBB respectively, in order to identify the exact components that extracted from bio-oil to biodiesel. Firstly, the FTIR spectra of bio-oil were tested and shown in Fig. 5. According to the locations of various absorption peaks, different radicals in bio-oil were identified. Combining the identification of variant radicals, different components in bio-oil were then identified as shown in Table 5 [41]. There were water, alcohols, ethers, ketones, carboxylic acids and furfurals in bio-oil. Secondly, the FTIR spectra of biodiesel and EPBB were tested and
3.2.1. Densities Fig. 7 shows the densities of biodiesel and EPBB at different temperatures. The densities of biodiesel and EPBB showed similar variation trend with the temperature. They decreased linearly with the increase of temperature at a same rate. This was because that EPBB was mainly consisted of biodiesel (about 80 wt%). At the same temperature, the density of EPBB was slightly higher than that of biodiesel. This indicated that not only the light constituents such as alcohols, ethers, but also carboxylic acids and some macromolecular compounds with higher densities as compared with biodiesel were also extracted from bio-oil to biodiesel.
3.2.2. Viscosities Fig. 8 shows the variations of the viscosities of biodiesel and EPBB with temperature. The viscosities of biodiesel and EPBB decreased with the increase of temperature at a same rate. When the temperature remained unchanged, the viscosity of EPBB was lower than that of biodiesel. This was due to that most of the constituents extracted from bio-oil to biodiesel have lower viscosities than biodiesel.
3.2.3. Surface tensions Fig. 9 presents the surface tensions of biodiesel and EPBB at different temperatures. The surface tensions of biodiesel and EPBB decreased at a same rate as the temperature rose. At the same temperature, the surface tension of EPBB was slightly lower than that of biodiesel. This was also due to that the surface tensions of the constituents extracted from bio-oil were lower than that of biodiesel.
3.2.4. Lower heating values Table 6 shows the lower heating values (LHV) of bio-oil, biodiesel and EPBB. Although the LHV of bio-oil was much lower than that of biodiesel, the LHV of EPBB was slightly lower than that of biodiesel. The reason was that a higher percentage of the constituents extracted from bio-oils have smaller lower heating value, while EPBB was mainly consisted of biodiesel.
3.3. Engine performance
Fig. 5. FTIR spectra of bio-oil.
3.3.1. Combustion characteristics The combustion characteristics were analyzed in terms of the in-cylinder pressure and temperature, heat release rate (HRR), ignition delay, premixed combustion and total combustion duration. HRR was obtained by calculating the following equation [16]:
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Table 5 Identification of the components in bio-oil by FTIR spectra. Components
Water
Alcohols
Radicals Absorption peaks
OeH 3200e3600
OeH 3200e3600
CeO 1050, 1080, 1265
Ethers
Ketones
CeO 1060e1150
C¼O 1715
Carboxylic Acids CeCeC 1100e1300
CeO 1395e1440
OeH 1210e1320
Carboxyl 1300-1420, 1550-1610-
Fig. 8. Variations of the viscosities of biodiesel and EPBB with temperature.
Fig. 6. FTIR spectra of biodiesel and EPBB.
Fig. 9. Variations of the surface tensions of biodiesel and EPBB with temperature.
Fig. 7. Variations of the densities of biodiesel and EPBB with temperature.
dp W p dV þ dQ þ R1cv V d4 þ p dV d4 d4 d4 dQB ¼ d4 1 l u cv T m vu 1 Hu vl mB dQW ¼ hc AðT TwÞ d4
Table 6 Lower heating values of bio-oil, biodiesel and EPBB. Fuel type Lower heating value/MJ$kg
1
Bio-oil
Biodiesel
EPBB
15.6
39.8
38.3
(2)
(3)
Where p, V and T are the pressure, volume and temperature of the in-cylinder charge. The heat transfer rate is given by Equation (3) and the heat transfer coefficient hc is determined from the
Woschni correlation. A and Tw are the area and average temperature of the heat transfer surface. Hu is the LHV of the fuel, relative air-to-fuel ratio l is the ratio of the actual air-to-fuel ratio to the stoichiometric ratio. m and mB are the mass of in-cylinder charge and the injected fuel. The definitions of the different phases for combustion analysis were mainly according to the methodology presented by Nour et al. [42]. Ignition delay was defined as the crank angle interval from the
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Fig. 10. In-cylinder pressure and heat release rate for biodiesel and EPBB. Fig. 11. In-cylinder temperature for biodiesel and EPBB.
start of fuel injection (SOI) to the start of combustion (SOC). The total combustion duration was the crank angle interval from SOC to the end of combustion (EOC). In detail, SOI was identified as the crank angle when the injector needle reached the 10% of its maximum lift. SOC and EOC were respectively identified as CA05 and CA90. Premixed combustion was identified as the crank angle interval from CA05 to CA50. CA05, CA50 and CA90 were the crank angles when the accumulated heat release rate was 5%, 50% and 90% of the total heat release rate, respectively. Fig. 10 and Fig. 11 shows the in-cylinder pressures and HRRs as well as in-cylinder temperature of the engine fueled with biodiesel and EPBB under the condition with BMEP at 0.42 MPa, and the engine speed at 1600 rpm and 2000 rpm respectively. In order to concentrate on the major combustion process, only a part of the data history was selected and shown. The curves start from fuel static delivery timing (25 oCA BTDC) to near EOC (40 oCA ATDC). Under both engine speed conditions, although the in-cylinder pressure and temperature rose later, they rose faster when EPBB was used instead of biodiesel. The curve tails of the in-cylinder pressure and temperature of EPBB finally become overlapped with those of biodiesel, which indicated a similar EOC for the two test fuels. In addition, the peak values of the in-cylinder pressure and temperature of EPBB were slightly higher than those of biodiesel at the engine speed of 1600 rpm, while they were slightly lower than those of biodiesel at 2000 rpm. The peak values of HRR
of EPBB were obviously higher than those of biodiesel under two engine speed conditions. It can also be inferred from the faster growth of in-cylinder pressure in Fig. 10 and the positive correlation between and the peak values of HRR and the pressure rise rate that the peak pressure rise rates of EPBB were also evidently higher than those of biodiesel [16]. Fig. 12 presents the ignition delay of biodiesel and EPBB under different BMEP conditions at the engine speed of 1600 rpm and 2000 rpm respectively. The ignition delay of EPBB was longer than that of biodiesel under all the conditions. The ignition delay consists of a physical delay and a chemical delay [42]. Generally, the physical delay is mainly influenced by the fuel density, viscosity and volatility, and the chemical delay is dependent on the fuel cetane number [43]. On the one hand, EPBB has the lower viscosity and surface tension (see Figs. 8 and 9) as well as the higher volatility (see Fig. 4) when compared with biodiesel. Therefore, the atomization and evaporation performance of EPBB was better. This better atomization and evaporation characteristic contraposed the effect of its higher vaporization enthalpy on the physical delay, and the proportion of physical delay was thereby minimized. On the other hand, the cetane number of EPBB was expected to be lower than that of biodiesel, due to the addition of the components with low cetane number in EPBB, such as alcohols, ketones etc. As a result, the chemical delay of EPBB was prolonged [43]. Overall, the
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chemical delay played a dominant role in the ignition delay, as such EPBB revealed a longer ignition delay as compared with biodiesel. The durations of premixed combustion and total combustion for biodiesel and EPBB under different BMEP conditions at 1600 rpm are shown in Fig. 13. As can be observed, the premixed combustion duration of EPBB were longer than that of biodiesel under all the conditions. This was mainly attributed to two factors. First, with the longer ignition delay of EPBB, more fuel was injected to the cylinder before the ignition. Second, the atomization and evaporation performance of EPBB was better as stated above. Under the coupling effects of these two factors, much air/fuel mixture was prepared for the premixed combustion. The premixed combustion duration of EPBB was thereby prolonged. Moreover, the total combustion duration of EPBB was however shorter than that of biodiesel under all the conditions. This was also mainly due to two reasons. On the one hand, the fuel atomization and evaporation of EPBB were better than those of biodiesel, so that the fuel/air mixing was promoted
Fig. 12. Ignition delays of biodiesel and EPBB. Fig. 14. BTEs of the engine fueled with biodiesel and EPBB.
Fig. 13. Premixed combustion duration and total combustion duration of biodiesel and EPBB.
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during the combustion. On the other hand, the oxygen content of EPBB was higher than that of biodiesel. The diffusion combustion of EPBB was improved as a result of the increase of fuel-bound oxygen at crucial zones. Hence, the whole combustion process was optimized and the total combustion duration was shortened. With a shorter total combustion duration, the EOC of EPBB was thereby about same to that of biodiesel despite of a longer ignition duration (see Fig. 11).
3.3.2. Fuel economy Fig. 14 shows the brake thermal efficiencies (BTE) of biodiesel and EPBB under different BMEP conditions at the engine speed of 1600 rpm. The BTE of EPBB was equivalent to that of biodiesel under all the conditions. BTE mainly depends on the indicated thermal efficiencies (ITE) and the mechanical efficiency, as described in the following formula:
het ¼ hit hm
(4)
Where het is the BTE, hit is the ITE, hm is the mechanical efficiency. As can be seen in the HRR curves in Fig. 10, the peak value of HRR was closer to the top dead center (TDC) when EPBB was used instead of biodiesel. This indicated that the combustion heat release of EPBB was more concentrated near the TDC, thus the ITE of EPBB were higher [44]. However, as concluded above in Fig. 10, EPBB has the higher peak values of the in-cylinder pressure and pressure rise rate, which represented a rougher combustion process and a lower mechanical efficiency. Under the coupling effects of these two factors, the BTEs of the two fuels showed minor difference. Fig. 15 shows the brake specific fuel consumptions (BSFC) of biodiesel and EPBB under different BMEP conditions at the engine speed of 1600 rpm. The BSFC of EPBB was slightly higher than that of biodiesel. BSFC is calculated from
be ¼
3:6 106 100% Hu het
(5)
where be is BSFC, Hu is the LHV of the fuel, het is the BTE. The BTEs of the two fuels were equivalent, while the LHV of EPBB was slightly lower than that of biodiesel as presented in Table 6. Thus, the BSFC of EPBB was higher than biodiesel.
Fig. 15. BSFCs of the engine fueled with biodiesel and EPBB.
Fig. 16. NOx emissions of biodiesel and EPBB under different loads.
3.3.3. Emission characteristics Fig. 16 presents the NOx emissions of biodiesel and EPBB under various loads at the engine speed of 1600 rpm and 2000 rpm. The NOx emission of EPBB was lower than that of biodiesel under all the conditions. It is well known that the formation of NOx in diesel engines is mainly according to the Zeldovich mechanism. The local flame temperature, high-temperature duration and the oxygen concentration are the main factors deciding the emission of NOx [45,46]. Since most of the constituents extracted from bio-oil have lower calorific value and higher enthalpy of evaporation when compared with biodiesel, the local flame temperature of EPBB was thereby lower than biodiesel. In addition, as can be seen from Fig. 11, the high temperature duration of EPBB was shorter than that of biodiesel. As such, EPBB revealed a lower NOx emissions despite of its higher oxygen content. The soot emissions of biodiesel and EPBB were characterized by the smoke opacity, as shown in Fig. 17. In comparison with biodiesel, the soot emission of EPBB was lower for most BMEP conditions. The emission of soot mainly depends on its formation and oxidation processes, where the latter process plays a major role. In compression ignition engines, soot is mainly formed by the partial oxidation and pyrolysis of the evaporate fuel under hightemperature and anoxic environment [47]. The atomization and evaporation performance of EPBB was better than that of biodiesel as stated before, so the anoxic region of over-rich mixture reduces and less soot was formed as a result. On the other hand, the addition of oxygen-enriched constituents from bio-oil increased the
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Y. Huang et al. / Renewable Energy 147 (2020) 1990e2002
Fig. 17. Soot emissions of biodiesel and EPBB under different loads.
oxygen content of EPBB, as such soot oxidation in the flame zone was promoted for EPBB [48]. Fig. 18 and Fig. 19 present the CO and UHC emissions of biodiesel and EPBB. The CO and UHC emissions of EPBB were respectively lower than those of biodiesel under all the conditions. The lower CO emission of EPBB was a predictable result, since the higher oxygen content of EPBB could reduce the region with over-rich mixture and suppress the incomplete combustion. On the other hand, the lower UHC emissions of EPBB was a surprising but welcome result. Generally, the longer ignition delay of EPBB leads to an increase of the over-lean region of air/fuel mixture, and thereby increases the UHC emissions. However, it can be inferred from the test results that the ignition delay may not have the leading role in UHC emissions of EPBB. The lower UHC emission of EPBB were likely attribute to the improvement of its atomization and evaporation performance. With this improvement, the regions with over-rich mixture were reduced, and the UHC emissions of EPBB were decreased despite of the increase of the over-lean region. 4. Conclusions In this paper, a pure biomass-based novel fuel, namely the extraction phase liquid of bio-oil/biodiesel blends (EPBB) was firstly
Fig. 18. CO emissions of biodiesel and EPBB under different loads.
prepared. In order to investigate its usability as an alternative engine fuel, the preparation efficiency, molecular compositions, thermophysical properties and diesel engine performance of EPBB were successively tested. Based on the test results, following conclusions are summarized. 1. When the initial mass ratio of biodiesel to bio-oil is 5:5, 5% noctanol was added as a co-solvent, the extraction efficiency K of biodiesel reaches the maximum. The highest extraction efficiency is about 21%, so that EPBB consists of biodiesel (about 80 wt%) and the best fractions in bio-oil (about 20 wt%). The whole preparation process is very convenient, and the thermal stability of EPBB is high enough for it to be used as an engine fuel. 2. During the extraction process, alcohols, ethers, ketones, carboxylic acids and some macromolecular compounds are extracted from bio-oil to biodiesel. As a result, EPBB shows a higher density, lower viscosity, surface tension and LHV when compared with biodiesel. The atomization and evaporation performance of EPBB is thereby better than those of biodiesel.
Y. Huang et al. / Renewable Energy 147 (2020) 1990e2002
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[3]
[4] [5] [6] [7]
[8]
[9] [10] [11] [12]
[13] [14]
[15]
[16]
[17]
[18] Fig. 19. UHC emissions of biodiesel and EPBB under different loads.
[19] [20]
3. In comparison with biodiesel, EPBB shows a longer ignition delay. Although the premixed combustion duration of EPBB is longer, its diffusion combustion and total combustion durations are shorter. The fuel economy of EPBB is comparable to that of biodiesel. More importantly, NOx, soot, CO and UHC emissions of EPBB are all more or less decreased when compared to biodiesel. 4. On the one hand, the poor fuel properties of the crude bio-oil obstruct its direct application as an engine fuel. On the other hand, the fuel properties and engine performance of EPBB are close or even better than those of biodiesel. Considering these two facts, it seems that preparing EPBB through the extraction of bio-oil by biodiesel will be a better choice to apply both bio-oil and biodiesel in diesel engines.
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Acknowledgements
[27]
The authors wish to express their deep thanks to the National Key Research and Development Program of China (No. 2018YFB0105900) for their support of the project.
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