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Biofuels and Biomass
Analysis of soot from the use of butanol blends in a Euro 6 diesel engine Magín Lapuerta, Jesús Sánchez-Valdepeñas, Javier Barba Salvador, David Fernández-Rodríguez, Juan P. Andres, and Tomas Garcia Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04083 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019
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Energy & Fuels
Analysis of soot from the use of butanol blends in a Euro 6 diesel engine Authors: Magín Lapuerta1, Jesús Sánchez-Valdepeñas1, Javier Barba Salvador1, David FernándezRodríguez1, Juan Pedro Andrés1, Tomás García2 Escuela Técnica Superior de Ingenieros Industriales. Universidad de Castilla – La Mancha. Avda. Camilo José Cela s/n. 13071 Ciudad Real, Spain. 1
2
Instituto de Carboquímica, ICB-CSIC, Miguel Luesma Castán 4, 50018, Zaragoza, Spain.
Corresponding Author: Magín Lapuerta E-mail:
[email protected] Telephone: +(34) 926295431 Fax: +(34) 926295361
Abstract The use of advanced fuels must increase from 0.5% to 3.6% of total fuel consumption in internal combustion engines according to the forthcoming European directive. In this frame, alcohols that can be obtained from waste or lignocellulosic materials with advanced production techniques may play an important role in the future. This work focuses on the effect of the use of butanol as a blend component on the properties of soot emmited from compression ignition engines. This knowledge is essential to decide the strategy to carry out a proper regeneration process in the Diesel Particle Filter (DPF). The study was performed in a Euro 6 diesel engine. The engine operating condition used to collect particulate matter was selected as a typical steady mode in urban driving. The blends tested were baseline diesel, Bu10D (10% butanol and 90% diesel v/v), Bu20D (20% butanol and 80% diesel v/v) and Bu10B10D (10% butanol, 10% biodiesel and 80% diesel v/v). The techniques used to characterise the soot are X-Ray Difraction (XRD), Raman spectroscopy, Transmission Electron Microscopy (TEM), surface area analyser, X-ray Photoelectron Spectroscopy (XPS), Fourier-Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). Among the results shown, most of the structural parameters related to the soot carbon layers did not correlate with reactivity, while others such as the concentration of oxygenated functional groups, the surface area (both increasing with butanol content) and the average primary particle diameter (which is reduced for increasing butanol content) provided good consistency with soot reactivity obtained from TGA and DSC.
1 Introduction Today, the most important issue related to the use of vehicles is pollutant emissions, which have direct effect on human health1 and environment2. Nowadays, climatic change, mainly produced as a consequence of global warming, has been pointed out as one of the most important environmental problem. Carbon dioxide (CO2) emissions emitted from vehicles have been highlighted as one of the main contributors to global warming3. Diesel engines, due to their higher efficiency, emit less CO2 than spark ignition engines but, on the contrary, the use of diesel engines is usually associated with nitrogen oxides (NOx) and particulate matter (PM) emissions. To reduce the emissions adverse effect, more and more stringent standards have been implemented all over the world. In Europe, from 2014, light diesel vehicles must fulfil Euro 6 standard4, which set strict limits for NOx and particulate emissions. One of the strategies to reduce pollutant emissions is the use of firstgeneration biofuels and advanced fuels to replace, at least partially, fossil fuels. Although biodiesel is the most used biofuel in road transport, the new directive RED-II5 supports the use of advanced biofuels, such as alcohols, to the detriment of first-generation biofuels. Although ethanol has been often used as a fuel component in diesel vehicles6,7, n-butanol has better properties (higher cetane number, heating value, viscosity and lower flash point)8,9. Additionally, n-butanol can be considered as an advanced biofuel as far
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as it can be produced from waste or lignocellulosic materials10-12. Currently, butanol can be produce through ABE (acetone-n-butanol-ethanol) or IBE (isopropanol-n-butanol-ethanol) fermentation10,13. Another strategy to reduce pollutant emissions is the use of different aftertreatment systems to fulfil with the standards. In current diesel engines, nitrogen oxides emissions are reduced in NOx aftertreatment systems14, such as LNT (Lean NOx Trap) or SCR (Selective Catalytic Reduction). On the other hand, particulate emissions are trapped in particulate filter (DPF) to be subsequently eliminated in an oxidation process, known as regeneration process. This process is affected by different factors like exhaust gas composition, temperature and flow rate, filter characteristics, temperature and flow profiles through the filter channels, and physicochemical properties of soot15,16. The study of soot characteristics and its implications on soot oxidation process could reduce the adverse effect of the regeneration process on fuel consumption and on filter lifetime. The properties of soot emitted by a modern Euro 6 engine have been analyzed with different analysis techniques (nanostructural, chemical and thermal). Among the nanostructural analytical techniques, X-ray diffraction (XRD)15,17, Raman spectroscopy18,19, transmission electron microscopy (TEM)20,21 or electron energy loss spectroscopy (EELS)22,23 can be highlighted. Some researchers have tried to relate soot nanostructure with its reactivity, but consistent conclusions have rarely been reached. In fact, it has been observed that soot reactivity correlates positively with some morphological characteristics but negatively with others24. Soot reactivity is defined hereinafter as the soot ability to be oxidized under lower temperatures or at higher rates. As a rule, it is accepted in the literature22,25 that as the distance between carbon layers increases and the fringe length decreases, the soot surface is more prone to be oxidized, due to the weaker binding energy between planes and to the higher availability of carbon atoms at the edge site, respectively. A characteristic which could have a great impact on the soot reactivity is the primary particle size, which is inversely related to both the soot surface area and the oxygen accessibility to the carbonaceous substrate21. Different authors have also studied the functional groups adsorbed in soot surface with chemical analytical techniques such as those based in infrared spectroscopy (FTIR, DRIFTS or ATR)26-28, X-ray photoelectron spectroscopy (XPS)29,30, energy dispersive spectroscopy (EDS)31,32, nuclear magnetic resonance (NMR)33,34 or near edge X—ray absorption fine structure (NEXAFS)26,35. Many studies have investigated the oxygenated and aliphatic compounds on soot surface, since they are supposedly related to the soot reactivity16,26,36. Finally, in other investigations, soot has been analyzed with thermal analytical techniques. In these studies thermogravimetric analysis (TGA) or differential scanning calorimetry (DSC), among other techniques, have been used to assess soot reactivity, by quantifying either the temperature required by soot to be oxidized (subjected to a heating ramp)15,36 or the time needed to be oxidized (when subjected to an isotherm process)16,29. It is necessary to remark that obtaining conclusions about soot reactivity based on structural or chemical analysis results can lead to misleading conclusions. For this reason, to study the impact of soot characteristics on reactivity, a complete soot characterization at different levels (nano and microstructure, morphology, porosity, surface area and functional groups) is proposed in this work. Finally, some previous studies have analysed the effect of blending biofuels with diesel fuel on the soot morphology and reactivity. Among these, very few have analysed the effect of the ethanol content20,28,37, and only one has observed the effect of butanol port injection20.
2 Experimental setup 2.1 Engine and operation mode This study was carried out in a Euro 6 Nissan 1.5 dCi engine (model K9K). This engine is a four-cylinder, four-stroke, turbocharged, intercooled, common-rail direct-injection diesel engine, and it is equipped with double exhaust gas recirculation system (EGR), with cooled low-pressure EGR (LPEGR) and non-cooled high pressure EGR (HPEGR). The aftertreatment system comprises a diesel oxidation catalyst (DOC), a Lean NOx Trap (LNT) and a regenerative wall-flow-type diesel particle filter (DPF). The main characteristics of the engine are shown in Table 1. A rotating shaft was used to couple the engine to an asynchronous electric dynamometer (Schenck Dynas III LI 250), which controls the engine speed and torque. INCA PC software and ETAS ES 591.1 hardware were used for the communication and management of the electronic control unit (ECU) of the engine. The inlet air mass flowrate and the fuel consumption were measured with the internal engine sensors and registered with the INCA PC software. Through the communication between the ECU and the INCA PC it
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Energy & Fuels
is possible to perform the test with the different fuels under the same operative conditions (injection pressure, EGR valve position and injection position). Table 1: Diesel engine characteristics. Fuel injection system Cylinders Valves per cylinder Bore (mm) Stroke (mm) Compression ratio Displacement (cm3) Maximum power (kW) Maximum torque (Nm)
DI, common rail 4 2 76 80.5 15.5:1 1461 81 kW @ 4000 rpm 260 Nm @ 1750-2500 rpm
In order to collect soot, the gas flows through a 47 mm diameter filter with 1 µm thickness. The filters, placed in a filter holder, have a retention efficiency of 99.99% and include a teflon membrane on a polymethylpentene ring in order to increase the rigidity. A vacuum pump sucks the gas though the filter. The filter is then scrapped and the particulate matter is collected for analysis. Finally, after devolatilisation for an hour at 400ºC under inert atmosphere to remove the volatile fraction, soot samples were analysed with different techniques, which will be described in section 2.3. Figure 1 shows a scheme of the engine installation where the soot collection system and the thermophoretic sampling are highlighted with purple lines. LOW PRESSURE EGR VALVE
AIR FILTER
FLOWMETER
LOW PRESSURE EGR INTERCOOLER
DOC + LNT
DPF
INTAKE AIR
BACKPRESSURE VALVE
MUFFLER EXHAUST GAS
FUEL TANK INTERCOOLER
HIGH PRESSURE EGR VALVE
PORTAFILTER AND FILTER
FUEL GRAVIMETRIC BALANCE (AVL 733S)
VACUUM PUMP
Fuel temperature control
THERMOPHORETIC SAMPLING DEVICE
ASYNCHRONOUS BRAKE (SCHENCK DYNAS LI 250)
ELECTRONIC CONTROL UNIT (ECU) ENGINE COOLER
COMUNICATION HARDWARE ETAS ES 591.1
CONTROL AND ACQUISITION SYSTEM (INCA PC)
Figure 1: Scheme of the experimental installation. Different previous studies have analysed the effects of engine speed15,38 and load15,18,28,38,39,40 on the characteristics of soot particles, and therefore, the effect of butanol content is analysed here in a single engine mode. The operation mode selected is illustrative of urban driving conditions and it represents an acceleration from 15 km/h to 32 km/h. This engine condition reproduces a low load operation mode characterized by its high emission of particulate matter, resulting in a high contribution to the DPF loading, and because its relative low exhaust temperature, which prevents from spontaneous regeneration of soot in the DPF. The main characteristics of this operation mode are shown in Table 2.
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Table 2: Engine operation mode characteristics Measured variables Engine speed (rpm) Effective torque (Nm) Air flow (kg/h) Start of main injection (ºCA bTDC) Start of pilot 1 injection (ºCA aTDC) Start of pilot 2 injection (ºCA aTDC) Fuel injected during pilot 1 injection (µL/inj) Fuel injection during pilot 2 injection (µL/inj) Injection pressure (bar)
Operation mode 1521 66 56 2.82 3.88 16 1.12 1.10 550
2.2 Fuels In this study, diesel blends with 10 and 20 %v/v of n-butanol content (Bu10D and Bu20D, respectively) were tested. Bu10D blend was selected since the well-known 20-20-20 target forces to 10% of the energy in the transport sector comes from renewable fuels41. Besides, n-butanol is considered an advanced fuel (since it is produced through an acetone-n-butanol-ethanol (ABE) fermentation process), whose contribution to the transport sector in energy basis will increase from 0.5% to 3.6% in the future European Regulations5. The other blend (Bu20D) has also been selected considering future regulations, in which an increase in renewable fuels is expected. Higher n-butanol contents were discarded because the low cetane number and the high enthalpy of vaporization of n-butanol would lead to cold start problems as confirmed in the work of Lapuerta et al.42. Finally, a ternary blend (Bu10B10D) composed of biodiesel (10% v/v), nbutanol (10% v/v) and diesel (80% v/v) has also been selected considering that first generation biofuels and advanced fuels will coexist in the next years. The diesel fuel used in this study was supplied by the Spanish oil company Repsol. It has no oxygen content and was similar to many diesel fuels supplied by petrol stations in Europe in winter. The biodiesel fuel used was donated by the Spanish biodiesel company Bio Oils and was produced from soybean (around 80% w/w) and palm oils (around 20% w/w). The content in saturated esters amounted to 20.64 % w/w, which is not far from an average saturation content of biodiesel fuels produced in Europe. Butanol was supplied by Green Biologics Ltd., as a member of the Consortium of ButaNexT Project. The main properties and reference standard methods of pure fuels and blends tested are shown in Table 3. Table 3: Main properties of tested fuels. Properties Density at 15 ºC (kg/m3) Kinematic viscosity at 40 ºC (cSt) Lower heating value (MJ/kg) Average molecular formula C (%wt) H (%wt) O (%wt) Molecular weight (kg/kmol) Stoichiometric fuel/air ratio CFPP (ºC) Lubricity (WSD) (µm) Derived cetane number Purity
Method EN ISO 3675 EN ISO 3104
Diesel
Biodiesel
Butanol
Bu10D
Bu20D
Bu10B10D
842.0
883.5
811.5
836.5
833.5
841
3
4.19
2.27
2.61
2.51
2.73
UNE 51123
42.93
37.64
33.20
41.75
40.69
41.23
C15.05H27.61
C18.68H34.64O2
C4H10O
C12.49H23.54O0.023
C10.59H20.50O0.040
C12.68H23.91O0.37
86.74 13.26 0
77.08 11.92 11
64.86 13.52 21.62
84.62 13.29 2.09
82.49 13.31 4.20
83.62 13.14 3.24
208.20
291.26
74.12
177.13
153.98
181.96
1/14.51
1/12.50
1/11.15
1/14.18
1/13.85
1/13.96
-20
-1
