combustion of bio-oil ethanol blends at elevated pressure

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Fuel 87 (2008) 232–243

Combustion of bio-oil ethanol blends at elevated pressure

Dung Nguyen, Damon Honnery *

Laboratory for Turbulence Research in Aerospace and Combustion, Department of Mechanical Engineering, Monash University,

Clayton, VIC 3800, Australia

Received 9 August 2006; received in revised form 23 April 2007; accepted 24 April 2007Available online 25 May 2007

Abstract

This paper reports an investigation on the combustion performance of bio-oil/ethanol blends. Experiments were conducted in a con-stant volume vessel operating at a pressure of 25 bar and temperature 1100 K. Bio-oil produced via the fast pyrolysis of a spruce feed-stock was blended to ethanol to form three stable blends containing 10%, 20% and 40% bio-oil by weight. In addition, ethanol andstandard automotive grade diesel were tested as reference fuels. Measured vessel pressure was used in a single-zone heat release analysis,while two-colour optical pyrometry was used to investigate particle loading and temperature. Results show that for similar injections offuel energy, use of up to 20% bio-oil in ethanol has limited impact on the performance of ethanol while 40% bio-oil in ethanol producedinstability in the pressure trace near the end of the combustion process. Burning rates are similar for blends and ethanol. Addition of bio-oil to ethanol was found to increase combustion generated particle load, and this increased with bio-oil concentration, but remainedmuch lower than particle concentration in diesel. Addition of bio-oil also resulted in formation of char particles that appear as luminousclusters outside the boundary of the spray. This suggests these particles will cool rather than oxidize. The presence of unburnt char par-ticles in large numbers may have consequences for bio-oil as an alternative diesel fuel.Crown Copyright � 2007 Published by Elsevier Ltd. All rights reserved.

Keywords: Bio-oil; Ethanol; Elevated pressure

1. Introduction

Ethanol has long been considered an important alterna-tive fuel for engines because it solves a number of problemsfacing traditional crude based fuels such as emission ofgreenhouse gases and particulates. As a fuel for dieselengines, it suffers from a very low cetane number (around5, while diesel is typically 50), and consequently a highauto-ignition temperature (around 640 K while diesel isaround 500 K). Ethanol has a heating value of about60% that of diesel. Igniting ethanol in a 4-stroke compres-sion engine either requires use of a higher compressionratio, use of glow plugs or a catalyst. Modificationsrequired to 2-stroke engines are not as great. Despite this,ethanol has successfully been used in for example Sweden[1] as an alternative to diesel in bus fleets and in the US

0016-2361/$ - see front matter Crown Copyright � 2007 Published by Elsevie

doi:10.1016/j.fuel.2007.04.023

* Corresponding author. Tel.: +61 0 3 99051988.E-mail address: [email protected] (D. Honnery).

in light trucks. Use as a blend with diesel in the form ofan emulsion has also widely occurred [2].

Use of bio-oil in diesel engines is far more limited. Likeethanol, bio-oil, which is produced by fast pyrolysis of lig-nocellulosic biomass [3], is an oxygenated renewable fuelwith a low cetane number [4]. Its heating value is about60% that of ethanol, but its higher density compensatesfor this. Further problems present when attempting touse this fuel in engines [5] and diesel engines more specifi-cally [6] due to its higher viscosity and acidity, because ofthe tars and fine particles it often contains and the forma-tion of char and solid residues during its combustion.Following the direction of ethanol research, attempts havebeen made to overcome these problems by blending bio-oilwith diesel to form an emulsion [4,7,8]. While theseattempts met with some success, long term operation onthese fuels is yet to be demonstrated and the additional costof the surfactant required to stabilise the blend is a furtherbarrier to their use.

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Nomenclature

V volume of the vessel (m3)p pressure of the vessel (bar)T gas temperature (K)c ratio of specific heatCp specific heat capacity of mixture (kJ/kmol K)R gas constant (kJ/kmol K)dQn/dt net heat release rate (kJ/s)dQch/dt gross heat release rate (kJ/s)dQht/dt heat transfer rate to the vessel wall (kJ/s)a empirical constant

fv particulate volume fraction (ppm)L optical path length (lm)k wavelength (nm)LHV lower heating valueHHV higher heating valueSOI start of injectionEOI end of injectionQF average fuel energy supply ratePGHR peak gross heat release rate (kJ/s)Fr ratio of PGHR/QF

D. Nguyen, D. Honnery / Fuel 87 (2008) 232–243 233

Fast pyrolysis bio-oil and ethanol form a stable blendwhen mixed without using surfactants or additives.Research on these blends in engines to date is limited togas turbines [5] and their use in these engines has provedpositive. Blending bio-oil with ethanol will not overcomethe problems of direct ethanol use in unmodified dieselengines. However, operating engines modified to run onethanol with ethanol/bio-oil blends may overcome theproblems associated with pure bio-oil particularly as theyrelate to burning of the fuel. As with all new fuels, technicalissues relating to such things as fuel storage, material com-patibility, engine start-up/shut-down procedures and longterm operation still need to be considered.

This paper reports on a series of experiments under-taken on bio-oil/ethanol blends combusted in a constantvolume vessel designed to simulate diesel engine condi-tions. Three blends, containing 10%, 20% and 40% bio-oil are investigated as are commercial ethanol and automo-tive diesel for reference. Thermodynamic analysis is usedfor the interpretation of the combustion process whiletwo-colour optical pyrometry is used to determine the par-ticle and temperature distributions inside the sprays.

Fig. 1. Experiment arrangement: (a) vessel arrangem

2. Experiments

2.1. Facilities

The stainless steel combustion vessel used in these exper-iments has a cylindrical internal volume of 2.384 l, being150 mm in diameter and 132 mm deep, Fig. 1. Eight50 mm equi-spaced windows are located around the cylin-drical wall of the vessel, with a larger 70 mm window in thebase plate. These windows are designed for optical access.For this series of experiments only one window on the sideis fitted with glass while all the others are covered withblind flanges. Fuel is supplied to the vessel via a Boschcommon rail injector which is fitted with two types of noz-zles serving for two types of measurements. A five-hole(0.2 mm in diameter each hole) star-configuration nozzleis used for thermodynamic measurements while a single-hole (0.2 mm in diameter) straight type nozzle is used forpyrometry measurements. The injector is located in thecentre of the top plate. Its opening pressure can be variedbut 120 bar is used here. Argon is used to pressurize thefuel to the required delivery pressure which is monitored

ent, (b) optical arrangement, (c) field of view.

Fig. 2. Typical pressure curve showing the vessel preheating, fuel injectionat SOI and post fuel injection cooling.

234 D. Nguyen, D. Honnery / Fuel 87 (2008) 232–243

by a pressure transducer. This can be varied from the open-ing pressure up to 220 bar. The injection pressure used inthese experiments was fixed at 180 bar. This is low by mod-ern fuel injection standards; however it is sufficiently highenough to provide for atomisation of the fuel, althoughfuel penetration will be shorter and fuel droplets larger.

To achieve diesel engine like conditions in the constantvolume vessel, a chemical pre-heating process is used. Thisinvolves igniting in the vessel a known mixture of CO, Airand O2. Variation in the composition of the mixture allowsfor different pre-injection pressure/temperature combina-tions. The presence of CO2 renders the mixture differentto that typically found in a diesel engine except when a rel-atively high level of exhaust gas recirculation is used [9].Attached to the vessel are pressure and temperature trans-ducers to monitor and control the experiment. A GEDruck static pressure transducer model PMP 4311 is usedto monitor the pre-injection heating process and at therequired condition acts to trigger the fuel injection. APCB Piezotronics transient pressure transducer model112B11 records the vessel pressure rise for use in a firstlaw of thermodynamics analysis of the fuel burning pro-cess. The static transducer is a linear automotive researchgrade absolute pressure transducer with a traceable calibra-tion (R2 = 1). It is necessary to calibrate the transient trans-ducer against the static transducer. This calibration is doneby regression analysis to time response matched compo-nents of the static signal to transient signal. The processof optimizing the calibration coefficients between the twotransducers results in a standard deviation of 0.06 bar ofthe linear fit (R2 = 0.9997).

2.2. Heat release and heat transfer models

If the constant volume vessel, V, is treated as a singlecontrol volume, application of the first law of thermody-namics yields the following equation [10]:

dQn

dt¼ dQch

dt� dQht

dt¼ 1

c� 1V

dpdt: ð1Þ

Here dQn/dt is the net heat release rate from combustionof the fuel. The net rate is the difference between the grossheat release rate, dQch/dt, and the heat loss rate to the ves-sel walls, dQht/dt. The net heat release of the fuel can bedetermined directly from the pressure, p, provided the ra-tio of specific heats, c, is known. In a single-zone model,all the cylinder contents are assumed to have the same cand for the conditions used here c is a function of compo-sition and temperature. Fig. 2 illustrates a typical preheat-ing and fuel injection curve. The initial 0.5 s of heating tothe peak pressure results from the ignition of the CO, Airand O2 mixture. This is followed by cooling via heat lossto the vessel walls. Once the desired ignition conditionsare reached, SOI (start of injection), fuel is injected result-ing in the small pressure rise that follows. For any partic-ular experiment, the composition of the gases in the vessel

is assumed fixed once the chemical pre-heating reactionhas reached equilibrium. During the following coolingprocess (Fig. 2), c thus becomes only a function of temper-ature. The assumption of fixed vessel gas composition canbe justified on the basis of the very small amount of fuelinjected into the vessel (F/A = 0.0025). The functionc = f(T) can be derived from the polynomial form depen-dence of Cp/R on the gas temperature using the Chemkindatabase [11].

Heat loss to the vessel walls will continue to occur dur-ing the fuel injection process. Calculation of the gross heatrelease rate requires this loss to be accounted for. This isdone by fitting a function to the heat transfer rate requiredto maintain constant internal energy equal to that at condi-tions equivalent to those at the point of fuel injection. Thefunction used is based on a convection model, but with anexponential heat transfer coefficient:

dQht ¼AeBT

TðT � T 0Þdt: ð2Þ

The coefficients A and B are found to be respectively0.217 · 10�2 W and 0.144 · 10�2 K�1 which yield a maxi-mum standard deviation of 0.02 kJ. Fig. 3 shows the appli-cation of this model to a typical fuel run.

Two main factors that affect the accuracy of the thermo-dynamic analysis results are the uncertainty in the calibra-tion between the two pressure transducers and theuncertainty in the heat transfer model. Typical variationof 0.06 bar from the mentioned calibration between thetwo pressure transducers is high compared to the pressurerise from the fuel combustion (typically less than 2 bar).This yields the maximum uncertainty in the pressure ofaround 6.7%. The very low injection pressure which leadsto a very small pressure rise from the fuel combustion isthe main reason for this problem. In combination withthe uncertainty in the heat transfer model, the uncertaintyin the results can be even higher. However, the resultsobtained are repeatable and this suggests an acceptablethermodynamic analysis of the fuel combustion process.

Fig. 3. Application of the heat release model to the net heat release to calculate the gross heat release from the fuel injected at SOI. Insert shows anenlarged view of the fuel injection process.


2.3. Two-colour pyrometry

The temperature and concentration of combustion gen-erated particles in the sprays are measured using two-col-our optical pyrometry. This is a well known techniquethat makes use of the thermal radiation emitted from theseparticles to calculate, via the Plank blackbody function andthe particle emissivity, the particle temperature [12,13]. Themonochromatic emissivity of the particles required in thismethod can be calculated via a number of methods [14].Here it is calculated from the empirical formula of Hotteland Broughton [15] using a constant of a = 0.95 fromLiebert and Hibbard [16]. Extension of the two-colourtechnique method via Mie and Lorentz–Mie theory enablescalculation of the particle volume fraction fv. The complexrefractive index required to do this is taken from Lee andTien [17]. Calculation of fv requires the optical path lengthof the measurement, L. In engines this is typically taken tobe a dimension characteristic of the combustion chamberand use of this will yield the average fv along the opticalpath. For the sprays examined here, calculation of fv

requires L along the line of sight normal to the plane ofthe spray. Determining this from instantaneous images isdifficult and prone to significant error, particularly nearthe edge of the spray where the path is small. As a conse-quence, the product fvL is reported as being representativeof the particulate loading.

The optical arrangement used is shown in Fig. 1. Two12-bit monochrome PCO Pixelfly cameras with CCD arrayof 1280 · 1024 px2 and exposure time range of 5 ls–65 sare used to record the intensity signals of the emitted ther-mal radiation. Fixed to these cameras are two Micro-Nik-kor 105 mm f/2.8D lenses. Two monochromatic narrowband pass filters chosen have centered wavelengths ofk1 = 701 nm and k2 = 452 nm with ±10 nm band pass.The field of view through the vessel window of each camerarelative to the nozzle tip of the fuel injector is also shown inFig. 1 (shaded region between the window and the CCDcorresponding area). Image collection triggering is con-trolled via an ECU.

To reduce the time for processing the data, the camerasare set to 2 · 2 binning mode; the size of images is therefore640 · 512 px2. With this setting, the pixel resolution is stillhigh enough to feature the flame [13]. For all experiments,the aperture of each lens is set to fully open. Because dieselhas high luminosity whereas bio-oil/ethanol blends havevery low luminosity, experiments required use of differentcamera integration times (diesel with 0.03 ms while blendswith 0.5 ms). An 18A GE Precision tungsten-filamentlamp, calibrated using a CIT optical pyrometer, is usedfor verifying the linearity of the cameras over the tempera-ture range investigated. It is also used to calibrate theresponse of each camera [12,13] for the different settings.The emissivity of tungsten at each wavelength and temper-ature required for this was taken from Latyev et al. [18].

A beam splitter was used so both cameras can image thesame region of the spray. Here each camera can be adjustedto have almost perfect coincidence of the images. The max-imum deviation between two images was about 0.1 pixelswhich is equivalent to 0.0078 mm in the image plane. Thiswas determined from a reference image correlation processsimilar to that used in digital PIV analysis [19]. The size ofthe imaged region is 50 · 40 mm2.

In the sprays examined here, the combustion generatedparticles will be produced by either soot or char formationprocess. The particles in the diesel sprays will consist exclu-sively of soot, while the fuel blends may contain both,although char is expected to dominate. The two-colourtechnique is not able to distinguish between particle types,but the presence of the larger char particles will alter theaccuracy of this measurement technique. Uncertaintiesarising from this technique are mainly classified into twotypes, those from particle properties and those from theexperimental system. Factors associated with particle prop-erties include optical properties, the emissivity model, sizedistribution and shape, measurement wavelength pair,and H/C ratio of the fuel. According to the analysis in[14], use of the semi-empirical formula of Hottel andBroughton (for k1 = 701 nm, k2 = 452 nm) will give a tem-perature uncertainty less than 10 K for diesel and 70 K for


ethanol/bio-oil blends; lower particle concentration typi-cally corresponds to greater uncertainly [14]. Use of thesoot refractive index leads to an uncertainty in fvL of about50% for diesel and perhaps over 90% for fuel blends. Whilethese fvL uncertainties are high, this technique can provideindicative differences between fuels for the same test condi-tions provided the effect of the differences in the fuel H/Cratio are relatively small [14], this is assumed here.

Experimental factors that affect the accuracy of theresults include, but are not limited to, the unwanted reflec-tions in the optical system and inside the combustionchamber, the background noise, and the deviation betweentwo images on the CCD arrays of the two cameras. In thissystem, the reflections come mainly from inside the com-bustion vessel while that from the optical system havealmost been eliminated within the setup. Determinationof the sensitivity of temperature and fvL on image deviationwas done by numerically shifting one image relative to theother by 1 px. The maximum temperature difference isabout 45 K for diesel and 35 K for blends. For fvL, themaximum difference is about 20% for diesel and 15% forblends. With a small deviation of 0.1 px between twoimages, the errors in the results from this source areexpected to be negligible. A temperature check has alsobeen performed using the tungsten lamp as the imagedintensity field. For both measurement settings, the calcu-lated temperature varied by up to a maximum of ±25 Krelative to that measured by the pyrometer.

3. Experimental conditions and fuels

The vessel temperature and pressure at the start of injec-tion were 1100 K and 25 bar. The vessel temperature wascalculated directly from the measured pressure and as suchrepresents the average temperature in the vessel. 1100 Kwas chosen to ensure that ignition delay of all fuels was suf-ficiently similar thus reducing the influence of cetane num-ber [20]. Use of chemical heating systems like that usedhere can give rise to temperature stratification in the vessel[20] during the cooling period before the fuel injection con-ditions are reached. Gas temperature measured in the ves-sel by fine wire thermocouples during the cooling phase atthe point of fuel injection and 90 mm lower revealed instan-taneous temperatures up to 200 K lower at the lower loca-tion. Comparison of the injection point temperature to thevessel temperature yielded a temperature on average 50 K

Table 1Fuel elemental analysis

Fuel type C (wt%) H (wt%) N (wt%) S

Diesel 86.40 12.80 <0.05 0Ethanol (E95) 49.56 12.95 0 0Bio-oil 41.70 7.55 0.2 �B10 46.70 11.14 <0.1 0B20 �46.55 �10.97 <0.1 �B40 �46.41 �10.79 <0.1 �

lower at the point of injection. Lack of thermocoupleresponse meant the actual temperature during the fuelinjection process could not be measured. Thus while thecondition is given as 1100 K, the actual temperature atthe point of injection is likely to be around 50 K lower.

Fuel blends tested were B10, B20 and B40 containing10%, 20% and 40% bio-oil in ethanol, respectively. Alsotested were commercial ethanol and automotive diesel forreference. For two-colour pyrometry measurements, thestraight type nozzle with single hole was used. Only diesel,B20 and B40 were tested due to ethanol and B10 havingtoo low a luminosity signal, i.e. they burn almost transpar-ently. For thermodynamic analysis all fuels were testedusing the five-hole star-configuration nozzle. The dischargecoefficient of the star nozzle was measured to be 0.38.

The bio-oil used to create the blends with ethanol wasproduced 6 months prior to these experiments via fastpyrolysis of a spruce feedstock at Aston University UK,using a bubbling fluidised bed pyrolysis reactor. This typeof reactor operates at about 400–500 �C using inert sandfor heating the feedstock which is finely ground and pneu-matically fed into the bed reactor by a high velocity nitro-gen stream. The lower heating value (LHV) of this fuel wasdifficult to determine by measurement. However the bio-oilelemental analysis for carbon, hydrogen, sulphur, nitrogenand oxygen was carried out and this enabled the calcula-tion of the higher heating value (HHV) of bio-oil by usingthe unified correlation of Channiwala and Parikh [21]. TheLHV can then be approximately calculated by subtractingthe HHV by the latent heat of condensation of water. Aswith many other bio-oils, this bio-oil is a dense, viscous,highly oxygenated fuel. No experiments were done usingpure bio-oil due to the difficulty of injection at the rela-tively low injection pressures used. Table 1 indicates theelemental components and the water content of the bio-oil and other tested fuels. The properties of these fuelsare listed in Table 2. For fuels other than the bio-oil, thecalorific values were derived from calorific bomb measure-ments. The viscosity of bio-oil is known to change duringstorage [22], even when in sealed containers at room tem-perature as it has been here, accordingly, the value listedin Table 2 was measured during the experiments. Whilenot measured, the viscosity of the blends is not exactly lin-ear with blend concentration [4] however a linear assump-tion will provide an indicative value. Other properties ofthe blends are predominantly mass weighted combinationsof the individual fuels.

(wt%) O (wt%) H/C ratio (by wt) Water (wt%)

.75-0.8 0 0.148 037.49 0.261 6

0.005 �50.50 0.179 22.40.0005 42.06 0.239 N/A0.0010 �42.38 �0.236 N/A0.0020 �42.70 �0.232 10.60

Table 2Properties of fuels

Fuel type LHV(MJ/kg)

Surface tension at 20 �C(dynes/cm)

Density at 20 �C(mg/ml)

Viscosity at 40 �C(cSt)

Gross heata

(kJ)QF

b

(kJ/ms)

Diesel 42.54 29.30 827 3.0 2.045 0.41Ethanol (E95) 25.89 22.30 786 1.08 1.218 0.244Bio-oil 16.57 34.88 1218 35.3 – –B10 24.96 N/A 833 5c 1.209 0.242B20 24.02 N/A 877 8c 1.193 0.239B40 22.16 N/A 952 15c 1.142 0.228

a Gross heat release from 5 ms fuel injection.b Average rate at which energy is supplied to vessel during fuel injection.c Indicative values.


4. Results and discussion

A general picture of the combustion of the fuels can begained from images of spray luminosity. Fig. 4 presentsimages of the luminosity of each fuel at 2 ms after SOI.Even with the different image integration times the lowerluminosity of the ethanol and blends are apparent. Lackof fine detail in all but the diesel image is a result of thelonger integration times. It is evident that addition of thebio-oil to ethanol does not change the basic structure ofthe flame as measured by the angle of the spray. Nor isthe basic structure of the diesel spray different to that ofthe ethanol and blends.

One very noticeable difference that accompanies theaddition of bio-oil is the presence of what appear to be

Fig. 4. Images of spray luminosity taken at 2 ms after

clusters of highly luminous particles lying outside theboundary of the spray. No evidence of these particles wasfound in either the diesel or ethanol sprays.

4.1. Thermodynamic analysis

Fig. 5 shows the vessel pressure traces, calculated grossheat release rates and cumulative gross heat release for die-sel, ethanol, B10 and B20. In this figure, the pressure isshown as the change relative to the vessel pressure atSOI. The injection duration of each fuel was 5 ms withend of injection marked EOI. While lower than ethanolin LHV (Table 2), the higher density of bio-oil compensatesfor this resulting in similar gross heat supply from the fuelsfor the ethanol and blends of 1.14–1.2 kJ. Heat supply

SOI. (a) Diesel; (b) ethanol; (c) B20 and (d) B40.

Fig. 5. (a) Pressure traces and (b) gross heat release rate and cumulativegross heat release for diesel, ethanol, B10 and B20. Time is referenced toSOI; pressure is referenced to vessel pressure at SOI.

Table 3Combustion efficiency, peak gross heat release rate (dQch/dtp) and ratio ofpeak gross heat release rate to fuel energy supply rate (Fr)

Fuels Combustion efficiency Quantities Values

Diesel 86% dQch/dtp (kJ/ms) 0.27Fr 0.66

Ethanol (E95) 88% dQch/dtp (kJ/ms) 0.20Fr 0.82

B10 92% dQch/dtp (kJ/ms) 0.20Fr 0.83

B20 96% dQch/dtp (kJ/ms) 0.21Fr 0.88

B40 87.2–91.7% dQch/dtp (kJ/ms) 0.196Fr 0.86


from diesel was highest at 2 kJ. The rate at which the fuelsupplies heat, QF, averaged over the 5 ms injection shownin Table 2, will follow these trends. Here, the calculationof QF is based on the average fuel supply rate. In keepingwith the higher heat supply diesel shows the highest pres-sure rise at about 1.8 bar, while ethanol, B10 and B20 arelower at about 1.1 bar. The similarity in blend gross heatsupply is evident from the similar peak pressures for thesefuels. For all fuels, gross cumulative heat release, Fig. 5b, isless than the total fuel energy supplied, and diesel has thebiggest relative loss. Expressed as a percentage of heat sup-plied, gross cumulative heat released ranges from 86% to96%, Table 3.

The lower cumulative gross heat release of each fuel rel-ative to the energy supplied is possibly a result of inefficientcombustion. As the heat transfer model used to calculatethe gross cumulative heat release does not have a thermalradiation term (this is because this model is established inthe pre-combustion period which involves almost no radi-

ation), this will contribute to the reduction in calculatedgross cumulative heat release. However in their constantvolume thermodynamic analysis, Anderson et al. [9] foundinclusion of a thermal radiation term still left around 10%of the supplied energy unaccounted for. Higher particleconcentration will result in greater radiative heat transfer.In comparison with diesel, ethanol, B10 and B20 havelower reductions in calculated cumulative heat releasewhich indicates either higher fuel combustion efficiencyor lower radiative losses or even both. However, betweenethanol, B10 and B20, the fact that B10 and B20 show lessloss in gross cumulative heat than ethanol suggests highercombustion efficiency for these blends than ethanol as theseblends not only have slightly lower LHV than ethanol butalso are expected to have higher radiative losses due totheir lower H/C ratio [23], Table 1. These observationsare supported by the two-colour pyrometry measurementsthat follow.

For B40, the calculated cumulative gross heat releaseand heat release rate of three repeated experimental runsare shown in Fig. 6. It can be seen clearly in this figure thatinstability occurs at the end of the combustion process,quite away from EOI. This instability was not found inrepeated measurements of the cumulative gross heat releaseof the other fuels. The instability of B40 in the post com-bustion process might be a result of the poor atomisationof this blend that accompanies increasing fuel viscosity,particularly for the 180 bar fuel injection pressure used.Operation of the injector with 40% bio-oil required regularcleaning to avoid jamming.

The duration of the combustion of diesel at 18–20 ms ismuch longer than the typically 15 ms of ethanol and theblends. Once ignited, ethanol is known to have a fast burn-ing rate [24], and addition of bio-oil does not appear toalter this. There is no evidence of any influence of thelow cetane number of the ethanol and blends on ignitiondelay for the vessel temperature used and this is consistentwith the ignition delay investigations of Shihadeh andHochgreb [25]. The ignition delay for all fuels is similarranging from 0.6 ms to 0.7 ms, Fig. 5. The short ignitiondelay and lack of rapid pressure rise indicates that all fuels

Fig. 6. (a) Cumulative gross heat release and (b) gross heat release rate ofthree experimental runs for B40. Time is referenced to SOI.

Fig. 7. Gross heat release rate and gross cumulative heat release forethanol injected for a period of 4 ms, 5 ms and 6 ms. Time is referenced toSOI.

Fig. 8. Gross heat release rate and gross cumulative heat release for B20injected for a period of 4 ms, 5 ms and 6 ms. Time is referenced to SOI.


are undergoing diffusion burning and all appear to havesimilar burning rates for the first 4 ms of the injection.Table 3 shows the typical measured peak gross heat releaserate (PGHR) and the ratio of PGHR to the fuel energysupply rate, Fr = (PGHR/QF). For all fuels, PGHR occursafter EOI, but for diesel the greater proportion of fuelburns once the 5 ms injection has ended. The PGHR of die-sel is higher than that of other fuels and this is consistentwith the pure bio-oil engine pressure traces of Shihadehand Hochgreb [6,25]. However unlike the other fuels, thediesel peak rate is well below the rate of energy supply fromthe fuel, Fr = 0.66. The peak burning rates of ethanol andthe blends are close to the rate at which energy is suppliedby the fuel, with B20 at Fr = 0.88 being the highest. Thisfurther supports the case for diffusion burning of ethanoland blends. For confirmation, Figs. 7 and 8 show the burn-ing rates of ethanol and B20 at three different injectiondurations of 4 ms, 5 ms and 6 ms. The almost constantrates of gross heat release of the longer 6 ms injection of

ethanol and B20 are respectively 0.85–0.86 and 0.91–0.95the rate at which energy is supplied by the fuel. For dieselfor the range of injection durations used, this condition ishowever never met.

4.2. Two-colour pyrometry results

Fig. 9 shows selected images taken at 2 ms after SOI fordiesel, B20 and B40. It can be seen from this figure that theimage collection area did not allow for complete visualisa-tion of the entire length of the spray. After typically 1.7 msfrom SOI, the tip of the spray passed the lower edge of thewindow.

The measurements of fvL shown in Fig. 9a for diesel,used here as a reference fuel, fall within the range of valuesfound in other studies [26–29]. In the work by Akiyamaet al. [26], diesel particulate measurements were performedin a rapid compression machine operating at a fuel injection

Fig. 9. fvL (left) and temperature (right) distributions of (a) diesel; (b) B20 and (c) B40 at 2 ms delay.


condition of 830 K and 27 bar. Typical maximum particleloading measurements found in this study are equivalentto fvL = 0.4 lm, while those in the present study are0.6 lm. This difference is likely due to the higher tempera-ture used in this study [28]. While not shown here, the peakvalues of fvL for diesel were found to increase with evolu-tion of the flame. Particle temperature distributions arealso shown in Fig. 9 for each fuel. In comparison to thosefound in Akiyama et al. [26], the temperatures of diesel inthe present study are higher. The lower injection tempera-

ture used in their study resulted in a maximum temperaturenear 2400 K while the higher temperature used hereresulted in a maximum near 3000 K. The temperaturerange in the current study is similar to that found in dieselengine measurements [13,29].

Measurements of fvL for the blends show lower particleloadings than for diesel with B20 being typically 30 timeslower, and B40 15 times lower. The poor spatial resolutionof the bio-oil blend images does not allow direct compari-son of localised structures in the sprays. However, one

Fig. 11. Temperature distribution histograms of diesel, B20 and B40 at2 ms delay.


noticeable difference is that the bio-oil blends appear tohave greater particulate loading in the centre and towardsthe tip of the spray while for the diesel particles also appearat the edge of the spray in high concentrations. The particletemperatures shown in Fig. 9b and c for B20 and B40 mir-ror the fvL results. B20 with lower fvL has higher tempera-tures than B40. Histograms of fvL are shown in Fig. 10 forB20 and B40 for three different times after SOI, whileFig. 11 shows temperature histograms for three fuels at2ms after SOI. One noticeable difference between B20and B40 is a reverse in the fvL trend with time after SOI.For B20, at 1.3 ms there are fewer regions with lower fvL

than at 3.0 ms, while for B40 it is the reverse. The greaterbio-oil content of B40 is seen in Fig. 10b to not only resultin higher particle loadings, but as the spray develops thespread in the fvL distribution also grows. B20 while show-ing a widening of its fvL distribution with time, Fig. 10a,continues to remain relatively narrow. In keeping withthe greater spread in the B40 fvL than for B20, the temper-ature distribution for B40, Fig. 11, is narrower as a resultof greater thermal radiation from the higher particulateload. Diesel shows a bimodal temperature distribution.Examination of the diesel fvL image in Fig. 9a shows muchgreater clustering of the particulates in the spray than for

Fig. 10. fvL histogram of (a) B20 and (b) B40 at 1.3 ms, 2 ms and 3 msdelay.

the other fuels. These clusters are responsible for the lowertemperature component of the distribution.

The presence of luminous char particles in the B20 andB40 images is evident. These particles have been observedin the droplet experiments of Shaddix and Hardesty [30]and in the atmospheric slow pyrolysis bio-oil spray flamesof Stamatov et al. [31]. Distillation of the bio-oil used hererevealed a non-distillable residue equal to 30.5% of the ini-tial mass and this will scale with bio-oil content in theblends. Such values are typical of fast pyrolysis bio-oils[25] and are due to the high pyrolytic lignin content of thesefuels. During high temperature fuel droplet evaporation,formation of char from this lignin via pyrolysis is favouredover evaporation. Char formation processes in bio-oil fueldrops are known to result in both solid sphere and ceno-sphere like structures [30,32]. The solid sphere like particlestypically have dimensions proportional to the non-distilla-ble residue fraction of the bio-oil, while the cenospherestructures have dimensions more characteristic of the diam-eter of the fuel drop. The diameter of the particles in theseimages is estimated to be 90–120 lm for both blends. Forthis char particle size, sphere like residues would requirethe original fuel drop to be 250–400 lm depending on thebio-oil content of the original fuel droplet. Viscosity ofthe blends increases in proportion to bio-oil concentrationand this will act to increase average fuel drop size for theinjection temperatures of the fuel (20–30 �C) used here[25]. From the data of Hiroyasu and Kadota [33] fuel dropsin the range 250–400 lm lie at the upper edge of the distri-bution expected for the fuel injection pressure used here.Fuel drops 90–120 lm in diameter lie within the expectedrange. It is therefore possible that the char particles seenin these images are more cenosphere like than sphere likein structure as their size is more representative of theexpected droplet distribution. Water content in bio-oil isknown to promote rapid droplet bubbling and in somecases this can lead to droplet explosion [30]. However sincethe boiling point of ethanol is lower than water and is in


larger concentrations, its rapid evaporation and bubblingin the fuel drop may be the cause of this char particle struc-ture. In diesel engines, the much higher fuel injectionpressures these engines use produce fuel drops around25–40 lm [33] and if the char formation processes aresimilar to those here, char particles in engines are likelyto fall in this size range.

While difficult to determine from the temperature fieldsin Fig. 9, the temperature of these particles covers theentire measurement range suggesting an ongoing formationprocess rather than a single event. While continuous timeseries images could not be taken, images taken from differ-ent experiments at different times support this suggestion.Although they are not found in all images, the large cloudlike clusters of the particles in the B20 image at(R,X) = (10,10) mm are generally found to exist nearerthe nozzle. The smaller longitudinal group in the B40image that lies along the edge of the spray (R,X) = (8,25)mm is more typical of those found far from the nozzle.Both types of particle groups appear in both B20 andB40 sprays. It is unclear if the downstream groupings arethe remains of the upstream particle clusters transportedto this region, or whether they have been transported tothis region from within the spray. Close examination ofthese images reveals what appear to be char particles inthe core of the spray. The presence of these particles welloutside the spray boundary will result in them cooling asthey are transported further from the spray. Once the tem-perature of these particles falls below 1400–1500 K, oxida-tion of these particles will cease. Although char particlesare expected to be smaller in a diesel engine, if these parti-cles are not completely oxidised, their presence in largenumbers could promote engine wear possibly leading toengine failure.

5. Conclusions

A number of conclusions can be made about the spraycombustion of the fast pyrolysis bio-oil/ethanol blendsexamined in this study. For the injection condition of1100 K and 25 bar used in these experiments, use of upto 20% bio-oil in ethanol has little effect on ethanol com-bustion performance. Peak pressures are found to beslightly higher, while burning rates for blends and ethanolare very similar and faster than standard automotive gradediesel fuel. Some evidence of higher combustion efficiencyfor the blends was found. For the blend containing 40%bio-oil, instability in the pressure, and consequently heatrelease, occurs at the end of the combustion process wellafter the end of injection. This may be a consequence ofthe high viscosity of this blend leading to its poor atomisa-tion. Higher fuel injection pressures than the 180 bar usedhere may improve this condition. Some difficulty wasencountered when injecting blends containing 40% bio-oil.

Measurement of the combustion generated particulateloading in the sprays revealed significant differencesbetween fuels. Particulate loading in the blends increased

with bio-oil concentration, but remained significantly lowerthan diesel. Higher particulate load was found to corre-spond to lower particle temperatures. While low luminosityof the blends resulted in poor spatial resolution, some dif-ferences in particle distribution between the fuels werefound. The blends show greater concentrations of particlesnear the centerline of the spray, while in diesel particleswere also found nearer the edge. One important differencebetween the fuels is the presence of large, highly luminouschar particles located outside the spray boundary of theblends. From an investigation of the size of these particles,it is possible that they are cenosphere rather than spherelike in structure. The location of these char particles out-side the boundary of the spray also suggests they will coolrather than oxidize and this could have consequences foruse of bio-oil as an alternative diesel fuel.

Acknowledgements

The authors would like to acknowledge the AustralianResearch Council (ARC) for funding the development ofthe facility used in these experiments and Prof. Tony Bridg-water, Aston University, UK, for supplying the bio-oil.The first author would also like to acknowledge the Viet-namese ministry of education and training (MOET) forfinancial support.

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combustion of bio-oil ethanol blends at elevated pressure

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