Article pubs.acs.org/EF
Evaluation and Treatment of Carbonyl Compounds and Fine Particles Emitted by Combustion of Biodiesels in a Generator Anne-Flore Cosseron,† Hayet Bennadji,† Gontrand Leyssens,† Lucie Coniglio,‡ T. Jean Daou,§ and Valérie Tschamber*,† †
Laboratoire Gestion des Risques et Environnement, Université de Haute Alsace, 3B, rue Alfred Werner, 68093 Mulhouse, France Laboratoire Réactions et Génie des Procédés, UPR CNRS 3349, Université de Lorraine, ENSIC-Nancy, 54000, France § Equipe Matériaux à Porosité Contrôlée (MPC), Institut de Science des Matériaux de Mulhouse (IS2M), LRC CNRS 7228-UHA, 3B, rue Alfred Werner, 68093 Mulhouse Cedex, France ‡
ABSTRACT: The main objectives of this study are the assessment of fine particles (PM2.5) and carbonyl compound emissions from an electrical generator fueled with biodiesels and the evaluation of the efficiency of a commercial 4-way catalytic converter toward these pollutants. Two different biodiesels were used: Soybean Oil fatty acid Methyl Esters (SOME) and Waste Cooking Oil fatty acid Ethyl Esters (WCOEE). Several biodiesel blends with petroleum diesel were tested (0%vol, 7%vol, 20%vol, and 50%vol). For all blends it was shown that particles are mainly composed of ultrafine particles with a diameter below 0.4 μm. The presence of a 4-way catalyst allows the reduction of the total number of particles emitted by at least 97%. However, the fraction of the smaller particles (PM0.1), which are particularly harmful to health, becomes predominant. The origin and concentration of biodiesel introduced in the fuel influence both the total number of emitted particles and their size distribution. The different behavior of the two biodiesels used in this study regarding particulate matter emissions is linked to the fuel properties. Interesting results are obtained from the measurement of carbonyl compounds emissions. This study reveals that the presence of biodiesel in the fuel has a major effect on acrolein, propanal, and acetone production. Moreover, it appears that the 4-way catalyst is not efficient toward carbonyl compound conversion.
1. INTRODUCTION The energy crisis undergone by the world in the last decades and the growing interest in air quality led the United States to search for alternative fuels to petroleum diesel for use in compression ignition engines. Fuels produced from biomass are promising substitutes since they are renewable, biodegradable, and potentially nontoxic. Commercial fuels in European countries currently contain 5.75% biodiesel and the target for 2020 is to reach 10%. Biodiesels are fatty acid esters (FAE) produced from the transesterification of vegetable oils or animal fats by addition of an alcohol. The feedstocks commonly used nowadays are edible vegetable oils and methanol; commercial biodiesels are thus named edible vegetable oil methyl esters (VOME). However, in order to reduce the competition with food market, it seems necessary to diversify the source of raw materials. Waste cooking oils (WCO) are an interesting feedstock as lipidic materials. Its use would significantly reduce biodiesel production cost and solve problems of soil and water contamination due to the fact that cooking oil is disposed in large amounts into the sewer systems of cities. Due to their composition (and thus their reactivity) which differs from petroleum diesel, the use of biodiesels in engines can affect exhaust emissions. Previous studies on this subject have shown that the impact of biodiesel combustion on pollutant emission factors depends on various operating conditions (type of engine, loading conditions, origin of biodiesel, rate of incorporation, etc.). However, most studies agree that the use of diesel fossil fuel mixture with either VOME 1 − 5 or WCO transesterified with methanol (WCOME)6−10 in diesel engine cars has an overall positive © 2012 American Chemical Society
effect on carbon monoxide (CO), total hydrocarbons (THC), and soot emissions. Increases of NOx emissions and fuel consumption are also frequently observed.2,5,11 However the results differ depending on the studies.1,3,12,13 Beside pollutants regulated by automotive standards (CO, NOx, soot, and THC), it is known that diesel engines produce non-negligible carbonyl compounds and fine particles with sizes lower than 2.5 μm in diameter (PM2.5), which are both harmful for environment and human health. Among studies recently published on the impact of biodiesel on carbonyl compounds emissions, wide discrepancies in results were observed. Some authors3,4,7,8 revealed that introduction of biodiesel in the fuel, whatever its origin, leads to a substantial increase of the emission of carbonyl compounds especially for light ones such formaldehyde and acrolein. Fontaras et al.,7,8 who investigated the effect of biodiesel origin on carbonyl emissions, concluded that, even if WCOME (produced from used frying oils) presents the worst impact, it is difficult to link carbonyl emissions to specific biofuel characteristics. Indeed, these authors highlighted that carbonyl compounds formation is highly dependent on the engine operating conditions. On the other hand, Correa et al.4 proposed that the increase of carbonyl compounds emission is a consequence of the oxygen content in the ester, while Peng et al.14 and Szybist et al.15 observed a beneficial effect of biodiesel in terms of aldehyde emission. These authors suggested later that the decomposition Received: June 12, 2012 Revised: September 12, 2012 Published: September 20, 2012 6160
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(WCOEE) in a petroleum diesel fuel (B0) is presented. The efficiency of a four-way catalytic converter, placed in the exhaust pipe, with respect to these pollutants is also studied. Results are then discussed in order to highlight the relationship among the fuel composition, the emissions of pollutants, and the reaction mechanisms that occur in the after-treatment.
of esters via decarboxylation decreases the probability of forming oxygenated combustion intermediates compared to petroleum diesel combustion. Regarding particle emissions, most investigations are consistent with a beneficial effect of biodiesel on both the mass of the emitted particles and the number of solid particles.9−13 The effect of biodiesel on the total number of particles, including volatile and nonvolatile fractions, is more variable and appears to depend both on the operating conditions of the engine (especially cold start or hot driving cycle and load conditions) and on the type of the biodiesel9,11,12 used. Lapuerta et al.,13 by comparing emissions from biodiesel fuels obtained by transesterification of waste cooking oil with either methanol or ethanol, observed that the mean particle diameter, the total particle number, and the smoke opacity decrease linearly with the oxygen concentration in the fuel. At the opposite, the mass of particles emitted from the combustion of ethyl esters is less sensitive to the oxygen fuel content than that of methyl esters. Fontaras et al.9 observed a decrease of the mass of the emitted particles when using WCOME compared to VOME. They linked their results with the saturated character of the biodiesel used. Thus, in addition to the influence of the oxygen concentration of the biodiesel used, these authors proposed that the higher saturated the fuel mixture is, the lower the particle emissions are. Surprisingly, Karavalakis et al.3 who studied the influence of the biodiesel origin in the same conditions as Fontaras et al.9 concluded that fuel blends composed of saturated biodiesel emit higher particulate matter (in mass unit). Thus biodiesel effects on the concentration and characteristics of particulate matter exhaust are not entirely clear. Among the numerous studies that investigated the effect of biodiesel nature on pollutant emissions, only a few of them tried to evaluate the impact of these new fuels on the efficiency of the after-treatment systems. Fontaras et al.9 assigned the increase in HC emissions observed when using biodiesel to the catalyst selectivity and concluded that oxidation after-treatment could perform differently than designed when using biodiesel. On the other hand, Young et al.10 proposed that the efficiency of diesel oxidation catalyst (DOC) associated with a diesel particulate filter (DPF) toward the nonvolatile fraction of particulate matter is not affected by the nature of the fuel. Automotive diesel engines are not the only diesel engines that generate air pollutants. Generators used to supply electricity at construction sites or to ensure an uninterrupted supply of power for critical installations also emit fine particles and carbonyl compounds. Despite their proximity to centers of human activity, studies on the environmental impact of diesel generators are rare because they are classified as nonroad sources and are thus not subjected to the regulation. However, the regulation is expected to change in the future. For this purpose, Pereira et al.16 showed that by using soybean oil biodiesel the electric energy generation is assured and a decrease in CO and HC emissions is observed. Valente et al.17 found that fuel consumption of a diesel power generator increases with the concentration of soybean or castor oil biodiesel introduced in the petroleum fuel. Moreover, according to these authors, all biodiesel blends tested produce higher CO and HC emission levels. In this paper, a comparative experimental study of carbonyl compounds and fine particles emissions of a diesel generator fueled with a mixture of fatty acid methyl esters from soybean oil (SOME) or fatty acid ethyl esters from waste cooking oil
2. EXPERIMENTAL SECTION 2.1. Fuel Production. To carry out this study, a petroleum diesel fuel (B0, provided by Total ACS, France) and blends of biodiesel with B0 (7%, 20%, and 50% v/v) referred to respectively as B7, B20, and B50 were used. Two types of biodiesel were tested. The first one is composed of fatty acid methyl esters (FAME) of soybean oil (named in the following SOME) from Novance (Compiègne, France). The second one, composed of fatty acid ethyl esters (FAEE) of WCO (named in the following WCOEE), was produced in the LRGP laboratory (Nancy, France). The latter was obtained by transesterification (homogeneous catalysis) of waste cooking oil (WCO) with ethanol (EtOH) at atmospheric pressure (atmospheric ethanolysis). The WCO, supplied by ENSAIA (France) after pretreatment (heating up to 60 °C followed by filtration), was initially an oleic acid enriched-sunflower oil, similar to that selected by major fast food chains. The low contents of free fatty acids (FFA) and water observed in the used WCO (0.4 wt % and 0.06 wt %, respectively) allowed operating via alkaline catalysis with additional amounts of catalyst to compensate losses in soaps.18,19 Sodium ethoxide (EtONa) was selected as alkali catalyst in order to avoid additional in situ saponification or water formation.20,21 Anhydrous EtOH and EtONa, as well as standards for FAEE identification and quantification (ethyl esters of palmitic, stearic, oleic, linoleic, and linolenic acids, 1-decanol and methyl heptadecanoate for the internal standard method) were supplied by Sigma-Aldrich. All reagents were of analytical grade (EtOH ≥ 99.8%, EtONa 95%, methyl heptadecanoate 98%, 1-decanol ≥99.5%, FAEE standards 98%). A two-stage ethanolysis procedure with separation of the glycerol byproduct after the first stage was adopted in order to shift the chemical equilibrium of the reaction toward the FAEE formation, thus enhancing the yield in biodiesel.22−24 WCO ethanolysis was conducted in a 4-L jacked reactor made of borosilicate glass and equipped with a reflux condenser (operating conditions: 80 °C, 1 wt % EtONa by weight of WCO, stirring speed 250 rpm, EtOH:WCO molar ratio 6:1). After 2 h of ethanolysis to ensure almost complete conversion of the WCO (mass fraction in FAEE of the crude biodiesel obtained at this stage: 91.7 wt %, determined by GC-FID as described in further details below), the reactor was cooled and the two formed layers (a lowerphase rich in glycerol and an upper-phase rich in FAEE) were separated by sedimentation. After a preliminary distillation to recover the residual ethanol (80 °C, 1 atm), the FAEE-phase underwent a twostage purification: (i) first, a bubble-washing process to remove mainly the remaining catalyst25 (45 °C, 1 atm, hydrochloric acid 0.01 N in distilled water as washing solvent), and (ii) second, a distillation under vacuum (30 mbar) to eliminate the washwater and to recover the FAEE by limiting risks of thermal cracking and oxidation during the process. The final FAEE-based biodiesel product (WCOEE), collected as distillate (temperature range observed at the top of the column: 190−225 °C) was a clear liquid, almost colorless or slightly yellow. Acidity and acid index determined according to the European standard EN 14104 were found to be 0.40 wt % and 0.80 mg KOH/gsample, respectively. 2.2. Engine and Test Rig Setup. Combustion of petroleum diesel (B0) and biodiesel blends were performed in a diesel generator provided by Yanmar. Table 1 shows its main characteristics. A generator load of 3000 W (which corresponds to 88% of the maximum load) was tested using a bench of 500-W lamps. Figure 1 presents the scheme of the test rig set up used for this experimental study. The setup consists of a generator, a fuel tank, and an exhaust pipe equipped with a four-way catalytic after-treatment system. Sampling probes located upstream and downstream of the after-treatment system allowed the analysis of the pollutants in the exhaust gas. The 6161
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favored at higher temperatures. The catalyst formulation and coating were characterized by X-ray fluorescence analysis (XRF). Main components of the support material are Al2O3, SiO2, and ZrO2. The catalyst is composed of platinum as active sites for oxidation reaction, barium, ceria, and potassium as NOx trapping materials, and ceria for its oxygen storage capacity, a necessary property to promote soot oxidation. 2.4. Pollutant Analyses. For each fuel blend, particles and carbonyl compounds were measured upstream and downstream of the after-treatment system. The measurements were repeated three times to increase the reliability of the test results. The aerosol in the exhaust was analyzed using on-line particle sizing technique. The aerosol measurement system included a Fine Particle Sampler (FPS-4000 purchased from Dekati) for diluting and conditioning aerosol, as well as an Electrical Low Pressure Impactor (ELPI purchased from Dekati) to measure airborne real time particle size distribution and concentration in the size range of 30 nm to 10 μm. The aerosol sample was extracted from the exhaust by using a stainless steel heated line (120 °C). The sample was subsequently diluted in two stages. The first dilution stage occurred in a perforated tube. The sample was mixed with dilution air forced through small pores in the perforated tube wall. The primary dilution air was heated to 120 °C to prevent nucleation and condensation. The second dilution stage occurred in an ejector-type diluter, located downstream of the perforated tube diluter. The ejector diluter acted as a pump which draws the sample from the primary dilution stage and dilutes it further. The secondary diluted gas exiting the ejector diluter was always at ambient pressure and temperature. Dilution, temperatures, and pressures were measured in real-time by a control unit enabling dilution ratio calculation second-by-second, which directly takes into account the changes in raw sample properties. In this study diluting ratios of 10:1 to 15:1 were used. The diluted sample was then introduced into the cascade impactor system (ELPI) that separated the particle matter following aerodynamic equivalent cutoff diameter at 50% efficiency in twelve particle size fractions ranging from 30 nm to 10 μm. Before entering the impactor stages, particles were charged in a positive unipolar particle charger (corona charger) according to their Stockes diameter. After being charged by the corona charger, the atmospheric particles were introduced in the cascade impactor in order to be classified owing to their inertia and their aerodynamic diameter. A multistage electrometer enabled counting the charged aerosol
Table 1. Diesel Power Generator Characteristics parameter
value
number of cylinders displacement volume engine power maximum electrical power nominal power
1 0.296 L 4780 W 3400 W 3000 W
consumption of the fuel blends was evaluated using a gravimetric method. Temperature and pressure was followed before and after the after-treatment system and the gas flow was determined using a venture system. In our conditions (generator load of 3000 W) the gas temperature is equal to 380 and 330 °C upstream and downstream of the after-treatment system, respectively. 2.3. After-Treatment System. The 4-way catalytic converter, from Toyota, consisted of two porous monolithic honeycomb structures. The first one is a straight-through monolith composed of a NOx Storage Reduction catalyst (NSR). It is followed by a wall-flow monolith named Diesel Particulate NO x Reduction catalyst (DPNR).26 The DPNR is 2.0 L in volume and 118 cells/cm2 in cell density. Filter channels are alternatively plugged to force the gas flow through the porous walls where particles are trapped. The structure of this substrate is optimized for particles to move into the substrate pores and the NOx storage catalyst component is coated not only on the substrate but also inside the pores.27 Soot is captured as the gas flows through the pores in the wall and is burned by the part of NO2 which is not adsorbed in the NSR and excessive oxygen coming from the engine under lean conditions.26,28 Millet et al.29 investigated, at laboratory scale, the behavior of the DPNR in terms of reaction mechanisms involving CO, HC, NOx, O2, and soot. These authors showed that with a gas composition close to lean diesel environment (equivalence ratio ER = 0.3), CO and HC light-off temperatures are 175 and 180 °C, respectively. They highlighted the dependency of NOx storage on NO2 production and its inhibition due to CO oxidation at low temperature. Indeed, Millet et al.29 observed that NO2 formation reached a maximum at 360 °C and reductants in the lean feed, such as CO and C3H6, hindered NO2 formation. This result was explained by the competition among reduction of NO2 with CO, oxidation of NO by O2, and oxidation of CO by O2, the latter being
Figure 1. Scheme of the test rig setup. 6162
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particles. Current was simultaneously measured for the 12 impactor stages and directly converted by the electrometer in particles number and concentrations using mathematical algorithms.30,31 Particle number and size measurements with this particular sampling technology (ELPI) imply that the particle charging is a function of the Stockes diameter and their separation is a function of the aerodynamic diameter of particles. Sampling was set at a constant flow rate of 9.81 L.min−1. To analyze formaldehyde, acetaldehyde, acrolein, acetone, propanal, butyraldehyde, and benzaldehyde, exhaust gas was pumped and came first through a filter in order to eliminate soot from the effluent then through three impingers filled with 150 mL of a 2,4-dinitrophenylhydrazine (2,4-DNPH) solution at 1.23 g/L and put in sequence. Sampling was performed for 1 h at 1 L/min. The samples were then analyzed with high performance liquid chromatography (HPLC, Varian 3012) coupled with a UV detector system (Varian 3050, detection at 360 nm). The mobile phase was a mixture of CH3CN and H2O (60/40 v/v). For the separation a C18 column (Interchim Lichrosorb 250 mm × 4.6 mm × 5 μm) was used and maintained at 35 °C. The flow rate was 1 mL/min and the injection volume was 20 μL. Acetone, acrolein, and propionaldehyde were quantified together due to the difficulties in resolving the chromatographic peaks using an isocratic elution.
Table 3. Consumption and Exhaust Gas Flow
SOME
WCOEE
density (20 °C) (kg/L) flash point (°C) viscosity at 40 °C (cSt) gross heating value (MJ/kg) water content (ppm wt) %C (wt) %H (wt) %O (wt)
0.795 75 2.6 47.32 31 86.5 13.5
