Emission performance of lignin-derived cyclicoxygenates in a heavy-duty diesel engineZhou, L.; Boot, M.D.; Luijten, C.C.M.; Leermakers, C.A.J.; Dam, N.J.; de Goey, L.P.H.

Published in:SAE International Journal of Engines

DOI:10.4271/2012-01-1056

Published: 01/01/2012

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ABSTRACTIn earlier research, a new class of bio-fuels, so-called cyclicoxygenates, was reported to have a favorable impact on thesoot-NOx trade-off experience in diesel engines. In this paper,the soot-NOx trade-off is compared for two types of cyclicoxygenates. 2-phenyl ethanol has an aromatic andcyclohexane ethanol a saturated or aliphatic ring structure.Accordingly, the research is focused on the effect ofaromaticity on the aforementioned emissions trade-off. Thisresearch is relevant because, starting from lignin, a biomasscomponent with a complex poly-aromatic structure, theproduction of 2-phenyl ethanol requires less hydrogen andcan therefore be produced at lower cost than is the case forcyclohexane ethanol. The goal of this paper, realized bymeans of experiments on a modified DAF heavy-duty dieselengine, is to investigate whether or not the (potentiallyprohibitively) expensive hydrogenation step from 2-phenylethanol to cyclohexane ethanol has an added value from anemissions perspective. The results suggest that this is not thecase and hydrogenation therefore does not seem like aninteresting additional step in the production process.

INTRODUCTIONDiesel engines are known to offer good fuel economy andlow carbon dioxide (CO2) emissions. Unfortunately, thediesel engine is a source of soot and NOx emissions, both ofwhich are subject to legal limits because of their adverseeffects on the environment and human health. Moreover, sootemissions from diesel engines produce a significant fractionof urban airborne particulate matter, which has beensuspected to cause an increased incidence of respiratoryproblems and cancer. It is therefore of considerable interest toreduce soot escaping oxidation during the diesel combustionprocess. In order to meet the requirements of future

legislation, emissions of these substances, as well as those ofcarbon monoxide (CO) and unburnt hydrocarbons (HC), willhave to be reduced. This could be accomplished either byimproving the combustion process within the cylinder or byother means, for example by using alternative fuels withoxygenated compounds.

In order to realize lower NOx and soot emissions,improvements of fuel injection systems (e.g. rate shaping,multiple injections), optimized combustion chamber designsand exhaust gas recirculation (EGR) have been investigated[1,2,3]. Still, Diesel engines produce relatively high NOx andsoot emissions that require expensive and, often, fuelconsuming exhaust gas after treatment technology. This is thedriver for continuous research, heading towards alternativecombustion modes, which promise a drastic reduction ofengine-out emissions at still favorable fuel efficiency.

In addition, there have been many studies on fuel effects onexhaust emissions, especially with respect to soot [4, 5]. Theresults suggest that reductions in sulfur, density, distillationtemperature and aromatic content, in particular multi-ringvariants, lower soot emissions. Although fuel properties arethe main factors influencing engine performance andemissions, there are still many challenges to identify theoptimal fuel properties and their effects on engine behavior.In particular, many researchers have investigated therelationship between chemical properties of fuels andformation of pollutants. The pyrolysis and oxidation ofseveral light hydrocarbon fuels was investigated over a widetemperature range using shock tubes [6, 7]. The thermaldecomposition of several aliphatic hydrocarbon fuels wasalso investigated using a fluid reaction tube [8]. Theseauthors reported the formation mechanism of benzene ringsfrom the decomposed low-boiling point hydrocarbons.Among various fuel properties such as CN, distillation

Emission Performance of Lignin-Derived CyclicOxygenates in a Heavy-Duty Diesel Engine

2012-01-1056Published

04/16/2012

Lei Zhou, M.D. Boot, C.C.M. Luijten, C.A.J. Leermakers, N.J. Dam and L.P.H. de GoeyEindhoven University of Technology

Copyright © 2012 SAE International

doi:10.4271/2012-01-1056

characteristics, aromatic content, the aromatic content inparticular is known to have significant effect on NOx and sootemissions [14].

Because of the depletion of conventional oil, futuretransportation fuels are expected to contain an increasingportion of alternative fuel components. Consequently, adeeper understanding of the combustion process not only ofnormal diesel, but also of possible alternative fuels, could beof help in reaching stringent emission limits with maintainedefficiency. A number of studies have shown substantial sootreductions for biodiesel and biodiesel blended with petroleumdiesel [9, 10], relative to neat petroleum diesel. However,they also show a slight increase in NOx emissions. Furtherresearch has shown that the molecular structure of biodieselcould have a substantial impact on emissions. Recent studieson fuel properties, especially CN, indicate that thehomogeneity of the cylinder charge is related to the CN ofdiesel or diesel-like fuels [11]. Low CN fuels that areresistant to ignite create longer ignition delays, therebyresulting in extended mixing time for fuel and air [12, 13].Yet, the strong resistance to auto-ignition of the low CN fuelsmay cause difficulties in certain combustion conditions, suchas cold start [14].

A strong connection between fuel molecular structure and thesoot formation mechanism during diesel combustion has beenshown and discussed in some fundamental studies [15,16,17].Basic studies on soot formation in laminar diffusion flameshave revealed that the sooting tendency of most hydrocarbonsis profoundly affected by the different types of carbon-carbonbonds [15]. In general, compared with straight chainhydrocarbons, soot production of fuels with aromatic ringsand triple-bonded carbon-carbon are relatively high[15,16,17]. While some studies [18,19,20] have focused onthe effect of aromatics on engine exhaust emissions,variations of aromatic content are often accompanied bychanges in other fuel properties, such as oxygen fraction, CN,boiling behavior, viscosity and heating value. It is thereforequite difficult to isolate the effect of aromaticity as such withrespect to engine performance and pollutant emissions.

Starting from lignin, which has a complex poly-aromaticstructure, the production of aromatic compounds (e.g. 2-phenyl ethanol) requires less hydrogen and can therefore beproduced at lower cost than is the case for saturated oraliphatic compounds (e.g. cyclohexane ethanol). The goal ofthis paper is to investigate whether or not the hydrogenationstep from 2-phenyl ethanol to cyclohexane ethanol has anadded value with respect to fuel economy and/or the soot-NOx trade-off.

MATERIALS AND METHODSFUEL PREPARATIONThe physical properties and heating values of the neat basefuel and oxygenates are listed in the table below.

As shown in Table 2, the CN of the cyclic oxygenates hasbeen derived from the measured MON (ASTM D2700 [22])and RON (ASTM2699 [23]) values of blends with gasoline.First, the MON and RON values of the pure oxygenates arecalculated according to Kay's mixing rule [24]. Finally, thecorresponding CN of these two fuels follows from equation(1) proposed by Kalghatgi [25].

(1)

Both oxygenates are blended to commercial diesel (EN590)to equal a fuel oxygen content of 2.33 (Table 3), as it is well-known that fuel oxygen heavily influences the soot-NOxtrade-off [14].

MEASUREMENT SETUPThe test engine, Cyclops [26], is a dedicated test rig, designedand built at the Eindhoven University of Technology. It isbased on a DAF XE 355 C engine. The specifications of thisengine are presented in Table.4. Cylinders 4 thru 6 operateunder the stock engine control unit, and together with awater-cooled, eddy-current Schenck W450 dynamometer,they are only used to start the engine and control therotational speed of the test cylinder, i.e. cylinder 1.

Table 1. Physical properties and heating value of the neat compounds

Table 2. CN derivation for the cyclic oxygenates

Table 3. Test blend properties

Table 4. Cyclops specifications

When data acquisition is idle, for instance during enginewarm-up or in between measurements, only the threepropelling cylinders are fired. Once warmed up and operatingat the desired engine speed, combustion phenomena andemission formation can be studied in the test cylinder. Apartfor the shared cam- and crankshaft and the lubrication andcoolant circuits, the test cylinder operates autonomously fromthe propelling cylinders. Fed by an Atlas Copco aircompressor, the intake air pressure of the test cylinder can beboosted up to 5 bar. The fresh air mass flow is measured witha Micro Motion Coriolis mass flow meter. Non-firingcylinders 2 and 3 function as EGR pumps (see Figure.1, theschematic layout of the setup). Their purpose is to generateadequate EGR flow, even at elevated charge pressures.

Figure 1. Cyclops engine test rig

Fueling of cylinder 1 is provided by a double-acting air-driven Resato HPU200-625-2 pump, which can deliver a fuelpressure up to 4200 bar. An accumulator is placed near (∼0.2 m) the fuel injector to mimic the volume of a typicalcommon rail and dampen pressure fluctuations originatingfrom the pump. The fuel mass flow is measured with a MicroMotion mass flow meter.

For measuring gaseous exhaust emissions, a Horiba Mexa7100 DEGR emission measurement system is used. Exhaustsmoke level (in Filter Smoke Number or FSN units) ismeasured using an AVL 415 smoke meter three times peroperating point, of which the average value is logged. In thispaper, we will refer to smoke as soot. Although it isacknowledged that the two are not the same, it is assumedthat the qualitative trends reported in this paper will hold alsofor soot nevertheless. The engine is equipped with allcommon engine sensors, such as intake and exhaust pressuresand temperatures, and oil and water temperature. This quasisteady-state engine data, together with air, fuel and EGRflows, as well as the regulated gaseous emissions, arerecorded at 20 Hz for a period of 40 seconds by means of anin-house data acquisition system (TUeDACS) [26]. Theaverage of these measurements is taken as the value for theoperating point under investigation.

Finally, a SMETEC Combi crank angle resolved dataacquisition system [12] is used to record and process in-cylinder pressure (measured with an AVL GU12C uncooledpressure transducer), intake pressure, fuel pressure andtemperature and injector current. All of these channels arelogged with a resolution of 0.1 °CA for 50 consecutivecycles. From this data, the average and standard deviation ofimportant combustion parameters, such as CA10, CA50 andIMEP, can be resolved online calculated online with theassociated SMETEC software.

ENGINE OPERATING POINTSAll tests are performed at 1200 rpm, with a fuel injectionpressure and temperature set to 1500 bar and 30 °C,respectively. As shown in Table 5, the operating points arebased on two intake pressure points with two mixturestrengths. Finally, injection timings (SOI), separated by 5°CA intervals, are chosen.

Table 5. Operating points

STOICHIOMETRIC RATIOCALIBRATIONBecause the compositions of the blends differ from normaldiesel, the stoichiometric ratio of each blend should becalibrated accordingly. The stoichiometric ratio of EN590diesel is roughly 14.3. The air composition is supposed to

contain 20.95% O2 and 78.09% N2, so that [N2]/[O2] =3.773, when other trace gases are neglected. If thehydrocarbon is described as CcHhOo, the combustion reactionequation can be written as

(2)

The stoichiometric ratio can then be written as

(3)

Furthermore, constants c, h and o of each blend can becalculated according to the volume fraction of thecompositions, resulting in the stoichiometric ratios listed inTable 6.

RESULTS AND DISCUSSIONSOOT AND NOX EMISSIONSGenerally speaking, soot in DI diesel engines is formed in thehot, fuel-rich core of the various jet-like burning diffusionflames. As shown in Figure 2, there is an overlap between theinjection and the combustion event for all fuels, even at theearliest SOI. At later SOI values, more towards TDC, thisoverlap shrinks and therefore the role of diffusion combustionbecomes less dominant.

As shown in Figures 3, 4, 5, 6, the soot emission of thesaturated cyclic blend is not lower, even higher in someoperation points than that of the corresponding aromaticblend. Moreover, when the SOI approaches TDC, thisdifference becomes larger. When combined, the aboveobservations suggest that when the diffusion phase of thediesel combustion becomes relatively more important, theimpact of fuel chemistry on soot is more profound.Considering that the combustion process, in particular thecombustion timing, is a key factor for soot formation in dieselengine, further analysis of combustion phasing will bediscussed in the next section.

Figure 2. In-cylinder pressure at SOI= −15 °CA aTDC,7.5 bar gross IMEP, no EGR and lambda=1.45

The use of EGR to reduce NOx emissions from diesel engineshas been investigated extensively before, for instance byLadommatos [27], who concluded that the most significanteffect of EGR is the reduction of the oxygen flow rate to theengine. This leads to a reduced local flame temperatureduring combustion and, thus, a reduced rate of (thermal) NOxformation. As shown in Figure 6, NOx levels indeed becomemuch lower when EGR is applied.

For each of the four operating points, all blends follow thesame trend (as compared to each other) as a function ofinjection timing. Normally, for the four operation pointsinvestigated, the soot emission of diesel is expected to behigher than that of both oxygenates, which is confirmed bythe graphs of soot emissions in the two operation pointswithout EGR (Figures 3 and 4), and the operation points oflow load with EGR in Figure 5. However, as shown in Figure6, in the operation point of high load with EGR, the sootemission of the two oxygenates is higher than that of normaldiesel. This might be explained by a lower combustiontemperature and associated poorer soot oxidation for the twooxygenate blends. This possible explanation is supported bythe lower NOx emissions seen for the oxygenated blends inthis work point.

Noteworthy is also that the soot trend in the work points withEGR differs from that in the work points without EGR.Specifically, the lowest soot emission in the operation pointwithout EGR is found for an SOI of 10 °CA BTDC or 5 °CABTDC. With EGR, 5 °CA BTDC is the SOI with the highestsoot. EGR thus seems to reverses the soot trend, but the

Table 6. Calculated stoichiometric ratio of the blends

relative soot ranking amongst the three fuels remainsunaffected. This suggests that aromaticity influences the sootemission, given that the fuel oxygen percentage in bothoxygenates is kept constant.

Since there is generally a trade-off between soot and NOx indiesel engines, it is interesting to compare soot with NOxtrends. As can be inferred from Figures 3, 4, 5, 6, thearomatic blend, containing 15% 2-phenyl ethanol, yieldscomparable trade-offs to the aliphatic cyclohexane ethanolblend. This suggests that hydrogenation does not appear to bea value-adding process step when producing fuel from lignin.

Figure 3. NOx vs. soot emission at 7.5 bar gross IMEP,no EGR, and lambda=1.45, for diesel (blue diamonds), 2-

phenyl ethanol 15% (green circles), and cyclohexaneethanol 17.1% (red squares)

Figure 4. NOx vs. soot emission at 12 bar gross IMEP,no EGR, and lambda=1.60, for diesel (blue diamonds), 2-

phenyl ethanol 15% (green circles), and cyclohexaneethanol 17.1% (red squares)

Figure 5. NOx vs. soot emission at 7.5 bar gross IMEP,30 wt% EGR, and lambda=1.00, for diesel (blue

diamonds), 2-phenyl ethanol 15% (green circles), andcyclohexane ethanol 17.1% (red squares)

Figure 6. NOx vs. soot emission at 12 bar gross IMEP,30 wt% EGR, and lambda=1.15, for diesel (blue

diamonds), 2-phenyl ethanol 15% (green circles), andcyclohexane ethanol 17.1% (red squares)

FUEL ECONOMY AND COMBUSTIONPHASINGIdeally, any improvement in the soot-NOx trade-off shouldbe realized without sacrificing fuel economy. However, ascan be seen in Figures 7, 8, 9, 10, fuel consumption for bothoxygenated blends is higher than that of diesel fuel in all offour operation points, with the corresponding standarddeviation of the ID over 50 engine cycles. In most of themeasurement points, the deviations of the results are less than0.5degCA (even smaller than 0.2degCA in most cases).

However, one should take into account that the heating valueof any fuel will inherently decrease with fuel oxygen content.Compared to the base fuel, the studied oxygenated blendshave a reduced gravimetric LHV of 2.7% and 1.9%,respectively (Table.3). This could be more or less expectedgiven the fuel oxygen content in the blends of 2.33 wt.-%.Based on this decrease in LHV, penalties in ISFC could beexpected in the order of 5-6 and 3-4 g/kWh for 2-phenylethanol and cyclohexane ethanol, respectively. As can beseen in figures 7, 8, 9, 10, 11, 12, 13, 14, this roughlyexplains the observed penalties in ISFC, suggesting that thethermal efficiency of combustion process is not significantlyaffected by the presence of the cyclic oxygenates at thestudied concentrations.

The fuel consumption of the aromatic blend is slightly higherthan (<1%) that of the saturated blends when the engine isrunning in an operating point without EGR. This could beexplained by the 0.7% lower LHV for the aromatic blend(table 3). The order of the fuel consumption appears reversedwhen EGR is utilized, except for the points withunexpectedly higher fuel consumption in Figure 11 atrelatively late SOI. There might have been a problem with theEGR flow rate control during the measurement (seeappendix).

As shown in Figures 7, 8, 9, 10, 11, 12, 13, 14, the ignitiondelays of the two kinds of oxygenates blends and referencediesel fuel are quite similar at high loads, with or withoutEGR. However, when the engine is running at low loads, thedifference in ignition delay becomes larger, especially at thepoints where injection timing is close to TDC when theengine running with EGR. This might be attributed to theaforementioned control problem of the EGR flow rate (seeappendix), which did not change the general trends of thesoot and NOx emission comparisons.

Figure 7. Fuel consumption (open symbols) and ignitiondelay (solid symbols), at 7.5 bar gross IMEP and no

EGR, for diesel (blue diamonds), 2-phenyl ethanol 15%(green circles), and cyclohexane ethanol 17.1% (red

squares)

Figure 8. Standard deviation of ignition delay, at 7.5 bargross IMEP and no EGR, for diesel (blue, right), 2-

phenyl ethanol 15% (green, middle), and cyclohexaneethanol 17.1% (red, left)

Figure 9. Fuel consumption (open symbols) and ignitiondelay (solid symbols), at 12 bar gross IMEP and no EGR,for diesel (blue diamonds), 2-phenyl ethanol 15% (greencircles), and cyclohexane ethanol 17.1% (red squares)

Figure 10. Standard deviation of ignition delay, at 12 bargross IMEP and no EGR, for diesel (blue, right), 2-

phenyl ethanol 15% (green, middle), and cyclohexaneethanol 17.1% (red, left)

Figure 11. Fuel consumption (open symbols) andignition delay (solid symbols), at 7.5 bar gross IMEP and

30 wt% EGR, for diesel (blue diamonds), 2-phenylethanol 15% (green circles), and cyclohexane ethanol

17.1% (red squares)

Figure 12. Standard deviation of ignition delay, at 7.5bar gross IMEP and 30 wt% EGR, for diesel (blue,right), 2-phenyl ethanol 15% (green, middle), and

cyclohexane ethanol 17.1% (red, left)

Figure 13. Fuel consumption (open symbols) andignition delay (solid symbols), at 12 bar gross IMEP and

30 wt% EGR, for diesel (blue diamonds), 2-phenylethanol 15% (green circles), and cyclohexane ethanol

17.1% (red squares)

Figure 14. Standard deviation of ignition delay, at 12 bargross IMEP and 30 wt% EGR, for diesel (blue, right), 2-

phenyl ethanol 15% (green, middle), and cyclohexaneethanol 17.1% (red, left)

SUMMARY AND CONCLUSIONSTwo cyclic oxygenates, 2-phenyl ethanol and cyclohexaneethanol, which could be synthesized from lignin, have beenblended to diesel fuel and used to investigate the impact ofoxygenate aromaticity on the soot-NOx trade-off and fueleconomy in a DAF heavy-duty diesel engine. The goal of thisstudy is to investigate whether or not the expensivehydrogenation step of 2-phenyl ethanol to cyclohexaneethanol is worthwhile from an engine performance point ofview.

Based on the results, a number of general conclusions may bedrawn:

Compared to diesel and for a given fuel injection timing, a1-3% higher ISFC is observed for the oxygenated blends.This penalty is roughly in line with the expected penalty of2.33%, based on the fuel oxygen content of both oxygenatedblends.

Compared to diesel and for a given fuel injection timing, alonger ignition delay can be observed for both oxygenatedblends. This can be explained by their lower CN (table 3).

The longer ignition delay, in turn, coincides and may be thecause of significantly lower soot emissions and slightlyelevated NOx due to a more prominent premixed combustion.

By also adapting the SOI to account for the longer ignitiondelay of the oxygenated blends, a significant simultaneousreduction in both soot and NOx can be realized, especially inoperating points without EGR and without an additionalpenalty in ISFC.

At an equal fuel oxygen content, the aromaticity of ligninderived (model) cyclic oxygenates does not appear to

significantly influence either the soot-NOx trade-off or fuelconsumption. This conclusion is of importance, given that thehydrogenation of the aromatic compounds found in lignin is acostly process.

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CONTACT INFORMATIONLei ZhouL.Zhou@tue.nlTel: +31402473731Fax: +31402433445Department of Mechanical Engineering CombustionTechnologyTechnische University EindhovenP.O.Box 513, 5600 MB Eindhoven, the Netherlands

ACKNOWLEDGMENTSThe authors thank the European Graduate School onSustainable Energy and KIC InnoEnergy for financial supportof this project. The authors also wish to acknowledge Bas vanden Berge and the technicians of the Eindhoven CombustionTechnology group: Bart van Pinxten, Hans van Griensven,Theo de Groot and Gerard van Hout, for their support withthe measurements.

DEFINITIONS/ABBREVIATIONSBTDC

before Top Dead Center

CA50Time of 50% Accumulation Heat Release

CA50Time of 50% Accumulation Heat Release

CNCetane Number

EGRExhaust Gas Recirculation

HHVHigh Heating Value

IMEPIndicated Mean Effective Pressure

IDIgnition Delay=CA10-SOI

ISFCIndicated Specific Fuel Consumption

LHVLower Heating Value

MONMotor Octane Number

RONResearch Octane Number

SOIStart of Injection

TDCTop Dead Center

According to Figure 9, there is a strange point of the ISFC and the corresponding ID at low load with EGR (in the red circle),However, this particular point was found to deviate because of a controlling problem of the EGR flow rate, as witnessed by theoxygen flow rate in the exhaust pipe as shown in Figure 11.

Figure. 11. Oxygen flow rate in exhaust, at 7.5 bar gross IMEP and 30 wt% EGR, for diesel (blue diamonds), 2-phenyl ethanol15% (green circles), and cyclohexane ethanol 17.1% (red squares)

APPENDIX

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