Environ. Sci. Technol. 2005, 39, 6792-6799
Effect of Water/Fuel Emulsions and a Cerium-Based Combustion Improver Additive on HD and LD Diesel Exhaust Emissions ARIANNA FARFALETTI, COVADONGA ASTORGA, GIORGIO MARTINI,* URBANO MANFREDI, ANNE MUELLER, MARIA REY, GIOVANNI DE SANTI, ALOIS KRASENBRINK, AND BO R. LARSEN* EU Joint Research Centre Ispra, Institute for Environment and Sustainability, Emissions and Health Unit, 21020 Ispra (VA), Italy
One of the major technological challenges for the transport sector is to cut emissions of particulate matter (PM) and nitrogen oxides (NOx) simultaneously from diesel vehicles to meet future emission standards and to reduce their contribution to the pollution of ambient air. Installation of particle filters in all existing diesel vehicles (for new vehicles, the feasibility is proven) is an efficient but expensive and complicated solution; thus other short-term alternatives have been proposed. It is well known that water/diesel (W/ D) emulsions with up to 20% water can reduce PM and NOx emissions in heavy-duty (HD) engines. The amount of water that can be used in emulsions for the technically more susceptible light-duty (LD) vehicles is much lower, due to risks of impairing engine performance and durability. The present study investigates the potential emission reductions of an experimental 6% W/D emulsion with EURO-3 LD diesel vehicles in comparison to a commercial 12% W/D emulsion with a EURO-3 HD engine and to a Ceriumbased combustion improver additive. For PM, the emulsions reduced the emissions with -32% for LD vehicles (mass/km) and -59% for the HD engine (mass/ kWh). However, NOx emissions remained unchanged, and emissions of other pollutants were actually increased for the LD vehicles with +26% for hydrocarbons (HC), +18% for CO, and +25% for PM-associated benzo[a]pyrene toxicity equivalents (TEQ). In contrast, CO (-32%), TEQ (-14%), and NOx (-6%) were reduced by the emulsion for the HD engine, and only hydrocarbons were slightly increased (+16%). Whereas the Cerium-based additive was inefficient in the HD engine for all emissions except for TEQ (-39%), it markedly reduced all emissions for the LD vehicles (PM -13%, CO -18%, HC -26%, TEQ -25%) except for NOx, which remained unchanged. The presented data indicate a strong potential for reductions in PM emissions from current diesel engines by optimizing the fuel composition. * Address correspondence to either author. Phone: +39-0332789293 (G. M., vehicle testing); +39-0332-789647 (B.R.L., chemical measurements). Fax: +39-0332-789259 (G.M.); +39-0332-789259 (B.R.L.). E-mail:
[email protected] (G.M.);
[email protected] (B.R.L.). 6792
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Introduction Particulate matter (PM) is a major environmental problem in urban environments. A number of studies have pointed to adverse health effects of particulate matter with a diameter below 10 µm (PM10). Limit values for PM mass concentrations in ambient air, expressed as PM10, are frequently exceeded in major cities, and the World Health Organization (WHO) has worked out that in Europe more than 100 000 people suffer premature death every year from effects of air pollution, with PM10 being of major importance (1). A recent major public health impact study of PM10 in 19 European cities, on a population of 32 million inhabitants, has estimated that reducing long-term exposure to PM10 concentrations by only 5 µg/m3 in these cities may prevent between 3300 and 7700 early deaths annually. (2). What is more, a recent major epidemiological study in 23 North-American metropolitan areas has demonstrated that PM10 pollution even plays a role in post neonatal infant mortality, such as the sudden death syndrome and respiratory diseases (3). Exhaust from heavy-duty (HD) and light-duty (LD) diesel vehicles is an important source of PM, and, even though emission standards have been designed to curb the pollution from these traffic sources, they are still counted as the major contributors to PM in urban environments (4). Epidemiological studies in eight major European cities conclude that cardiac admissions are likely to be mainly attributable to diesel exhaust (5). In the diesel sector, engine technology is in continuous development, and a number of after-treatment systems have been found, which, in combination with an enhanced fuel quality, reduce emissions of PM, such as the oxidative catalyst and the particle trap. Today, the oxidative catalyst is incorporated in all modern LD diesel vehicles. The particle trap is a technically feasible solution not yet implemented in all diesel vehicles. To meet future, stricter emission standards, the particle trap seems unavoidable for LD and could be one of the future options for HD vehicles. However, there is a notable time lag between the introduction of a new emission standard, or a new engine technology, and the renewal of the vehicle fleet, and, as a short term alternative to the particle trap, new fuel formulations may be employed to abate particle emissions. The emulsion of conventional diesel fuel and water, produced by addition of a small amount of water and an appropriate surfactant to the fuel, has recently gained popularity for HD diesel engines, especially in the public transport sector. The cooling effect obtained by the presence of water in the combustion chamber is particularly efficient in suppressing the formation of NOx and may also play a role for PM. Additionally, the improved nebulization in the combustion chamber during the injection phase, by so-called micro-explosions of the emulsion, and the delay it causes in the ignition may play a role. Finally, water may also have an influence on the chemical reactions forming soot (6-13). LD diesel vehicle emulsion formulations have been proposed. However, a reduced amount of water in the emulsion is a necessity for this type of engine to avoid impairing performance and durability. With the addition of less water to the emulsions for LD applications, it can be expected that gains obtained in emission reduction would be lower. To date, this has not been fully investigated. Another short-term alternative to the particle trap is the use of metalbased additives (14). For the present study, we have tested a cerium-containing multifunction additive package with Cetane improver, detergent, and a cold-flow properties improver. This additive represents the typical composition of similar products on the market and is designed to improve the combustion process (14, 15). 10.1021/es048345v CCC: $30.25
2005 American Chemical Society Published on Web 07/27/2005
Diesel exhaust contains a group of regulated constituents, that is, PM, carbon monoxide (CO), hydrocarbons (HC), and NOx, and a number of compounds that are not specified by the emission standards. The analysis of such unregulated compounds as individual species in diesel exhaust is important, either due to their carcinogenic properties, which is the case for polycyclic aromatic hydrocarbons (PAH, 16), or due to their potential for photochemical ozone formation, such as the volatile organic compounds (VOC, 17). Under the framework of collaboration between Regione Lombardia, Italy, and the European Union Joint Research Centre, the effect of alternative, reformulated, and modified fuels on pollutant emissions from diesel vehicles is investigated in our laboratories. In this paper, the effects of emulsions and fuel additives on emissions of PM, NOx, CO, PAH, and VOC from LD and HD diesel engines are studied. Emission factors are presented for each pollutant for a fleet of seven common EURO-3 LD diesel vehicles as mass per driven km on the dynamometer with the New European Driving Cycle (NEDC, 18) and as mass per produced kWh by a EURO-3 HD diesel engine (commonly found in the European HD fleet), during the European Stationary Cycle (ESC, 18) on the dynamic test bench.
Experimental Section Test Vehicles and Engines. Seven different EURO-3 LD diesel vehicles, with an approximate 2 L displacement and exhaust gas recirculation (EGR), were used and compared to a common EURO-2 LD diesel vehicle, representing the common rail system, the unit injector technology, and the old rotary pump technology (detailed information can be found in Table S1 of the Supporting Information). As representative of the European heavy-duty sector, a EURO-3 HD engine with a 10 L displacement 6 cylinder, no EGR, and a maximum power of 316 kW was selected. Test Fuels, Emulsions, and Additives. Whereas diesel/ water (D/W) emulsions for heavy-duty applications have been marketed for some years in various countries, D/W emulsions for light-duty applications are not currently available. In the present study, we used a commercial HD emulsion (GECAM) and two experimental emulsions specially formulated for LD vehicles. The HD emulsion contained 12% water. The LD emulsions were comprised of two commercial diesel fuels (fuel 1 and 2) and two experimental emulsions (detailed information can be found in Table S2 of the Supporting Information) with a water content of 6%. A larger PM reduction effect would be expected by using higher water content. However, preliminary tests have demonstrated that more than 6% blends significantly increase the risk of impairing engine performance and durability. The two base fuels of which the emulsions were made had very different physical properties: fuel 1 had a density close to the upper limit of the legislative range (0.820-0.845 g/cm3), while fuel 2 was close to the lower limit. Both fuels were low in sulfur content, especially fuel 1 (less than 10 ppm). The two fuels differed also in the distillation curve and for the aromatic content. The two emulsions were produced using the two base fuels and an identical formulation; however, the final blends of the two emulsions differed for the actual values of the Cetane improver and water content. The base fuel for the HD emulsion study (fuel 3) had a low content of sulfur, a relatively low density (0.826 kg/L), and a relatively high Cetane number (53.5). The addition of water resulted in an increased density (0.848 kg/L) and a decreased Cetane number, the later of which was countered by the additions of approximately 2 g of Cetane improver per kg of base fuel. Preliminary tests confirmed that it was not possible to reach the same power output with emulsions as with the water free base fuel. Hence, to ensure comparability of the emission results, the base fuel
was tested using the same ESC cycle derived from the maximum power curve of emulsions. For the additive studies, the test fuels (fuel 3 for HD and fuel 4 for LD) were commercial, low sulfur diesel fuels having a relatively low density and a relatively low content of aromatics as compared to the market average. The AMFALFA additive is made of two main components: 85% multifunction package (Cetane improver, cold properties enhancer, detergent) and 15% combustion improver catalyst, consisting of a mixture of organo-metallic compounds based mainly on Cerium. The additive was provided to the JRC with the two components separated; they were then blended and added to the fuel, following the recommendations of the producer. After the two components of the additive had been blended together, the additive was added to the fuel at 0.35% vol. Detailed information on the test fuels can be found in Table S2 of the Supporting Information. Measurements of Regulated Emissions. Regulated pollutant emissions from LD vehicles were measured using a chassis dynamometer and a conventional constant volume sampling (CVS) system with a critical flow Venturi. The CVS was equipped with four critical orifices that allow the selection of the most appropriate flow rate. The roller bench of the chassis dynamometer was a 48” single roller type. To follow the legislative cycle, the driver was assisted by a driver aid system. CO, NOx, and PM emissions were measured on diluted exhausts, while HC was measured continuously on raw exhaust using a heated sampling line. All exhaust sampling was carried out on diluted exhausts in a dilution tunnel (average dilution ratio approximately 100). The regulated emissions were measured as follows: carbon monoxide (CO) with a nondispersive infrared (NDIR) analyzer, total unburned hydrocarbons (HC) with a flame ionization detector (FID), and oxides of nitrogen (NOx) with a chemiluminiscense analyzer (CLA) using a NO2 to NO converter. Particulate samples were collected according to the legislative procedure for diesel vehicles using Tefloncoated glass fiber filters (Pallflex T60A20), and the mass was determined by weighing. Regulated pollutant emissions from the HD engine were measured using a full flow dilution tunnel and a CVS system. Particulate mass was measured by weighing, and the diluted exhaust was sampled from the secondary dilution tunnel. Measurements of Unregulated Emissions. The characterization of unregulated pollutants in diesel exhaust was carried out as described in detail in the Supportive Information and the cited references: the mass/size distribution was measured using a 12-stage low-pressure impactor (LPI); the particulate-associated PAH was determined by gas chromatography-mass spectrometry (GC-MS) based upon the methods EPA TO13 and the ISO/DIN 12844 (19) and validated by a successful participation in an interlaboratory comparison using the reference material (NIST SRM 1650 soot) and PM from vehicle exhaust (20). For data reduction, the toxicity equivalency factor (TEF) approach was used, in which each individual PAH is assigned a toxicity rating relative to benzo[a]pyrene that is set to unity. The benzo[a]pyrene toxicity equivalents (TEQ) of a given sample are calculated as the sum of the concentrations multiplied by TEF over all of the measured compounds (21). The TEF values used are listed in the Supporting Information; individual ozone precursor VOCs were sampled from the dilution tunnel and accumulated for each test-cycle in Tedlar bags and analyzed by dual column GC with flame ionization detection (22). A standard mixture obtained from the National Physics Laboratory (UK) containing known amounts of all determined VOCs (23) was used for response factor calculations and for determination of retention times. This mixture contains all 30 ozone precursor VOCs (C2-C9 hydrocarbons) specified in the European Ozone Air Quality Directive 2002/3/EC. The VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Results and Discussion
FIGURE 1. Effect (fleet average ( 95% confidence interval) of emulsion (dark gray) and combustion improver additive (light gray) on emissions from a EURO-3 HD diesel engine (top) and EURO-3 LD diesel vehicles (bottom). The t-test p-value is indicated, when lower than 0.05. Maximum Incremental Reactivity (MIR) approach (17) was used to estimate the potential impact of the emitted VOC mixture on ozone formation. Details on VOCs and MIR are given in the Supporting Information. Cerium was analyzed by high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) to study the fate of the metallic cerium contained in the combustion improver additive. The instrumentation and the sample preparation followed procedures described for ambient particulate elsewhere (24) with the acquisition settings for cerium (25). Statistical Analysis. The results of the measurements of regulated and unregulated emissions were subjected to analysis of variance (ANOVA) using the STATISTICA software version 6 from StatSoft (Tulsa, OK). For the HD study, individual ANOVAs were conducted with the factors “emulsion” (with/without) and “additive” (with/without) to analyze the effect on the emissions per produced kWh (PM/NOx/ CO/HC/TEQ). Each cell consisted of 3-4 repetitions of the emission measurement. For the LD study, individual ANOVAs were conducted with the factors “emulsion” (with/without) and “additive” (with/without) to analyze the effect on the emissions per driven km (PM/NOx/CO/HC/TEQ). In the emulsion study, the base fuel type (fuel 1/fuel 2) was used as an additional factor. The inter test-vehicle variation was 2-30 times higher than the intra test-vehicle variance; thus the average emission result for each test vehicle was used as repetitions in the ANOVA cells (n ) 3 for the emulsion study; n ) 4 for the additive study). All ANOVAs revealed significant interactive effects between factors, which imply that the effect of emulsion or additive was not the same for all types of emissions. Thus, it was necessary to conduct two-sided t-tests to evaluate the mean effect for each individual emission. No interactions were found for the factor “fuel type”, and thus data were pooled for fuel 1 and fuel 2 (n ) 6) before t-testing. In Figure 1, the results of the statistical tests are plotted as the mean effect of using emulsions or additive, the 95% confidence intervals as bars, and the t-test p-value when p is less or equal to 0.05. 6794
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All regulated emissions measured in the present study (Table 1) were below the limits of the EURO-3 standards, which for HD engines are 100 mg/kWh for PM, 660 mg/kWh for HC, 2100 mg/kWh for CO, and 5000 mg/kWh for NOx and for the LD vehicles are 50 mg/km for PM, 640 mg/km for CO, and 560 mg/km for HC + NOx. Not even in the cases where the use of W/D emulsions increased emissions were these limits exceeded. The emission factors for unregulated individual compounds measured in the present study using base fuel without emulsion or combustion improver additive are shown in Table 2. Effects of W/D Emulsion - HD Engine. The results on the regulated and unregulated emissions are presented in Table 1. The measurements were highly reproducible with coefficients of variance in the order of 1-2% for the regulated emissions and 4-13% for the TEQ emissions, which compares favorably with literature (e.g., 26, 27). The 95% confidence interval of the TEQ emissions was somewhat higher than the 10-18% variability typical for the analytical method for PAHs (19, 20), which also reflects variability in the combustion processes controlling PAH emissions. The effect of the emulsion on the emissions is shown in Figure 1. A significant drop in emissions was obtained for PM (-59%) and CO (-32%) together with a minor reduction in emissions of NOx (-6%). The only drawback was a slight increase in the emission of hydrocarbon (+16%). A number of studies have shown the same trend of beneficial effects by D/W emulsions on particulate emissions and NOx emissions, but with variability in the magnitude of the effect (7-12). For CO, however, no clear conclusion can be drawn from these studies. A very comprehensive study has been published recently by the U.S. Environmental Protection Agency (13) in which emissions data were obtained from a wide range of conditions including engine type and model year, on and off road applications, with and without after treatment emission controls. With the investigated commercial W/D emulsion (20% water), emissions of PM and NOx were reduced on average, by 58% and 14%, respectively, and ozone precursor reactive organic gas (ROG) emissions were increased by 87%. In a multi-media assessment of these results, it has been pointed out that the emissions of ROG by use of the W/D emulsion are about 29% of the NOx emissions in diesel exhaust, which in other words means that for each ton ROG increased, NOx will be reduced by 3.4 tons (28). Thus, when evaluating the emission effects on an absolute basis, mass emission reductions for NOx by use of emulsions was greater than mass emission increases of total HC. The PAH emission factors (Table 2) are lower than emissions we have previously found for a 10L EURO-3 HD D-DI engine (sum of PAH, 20 ( 1.1 µg/kWh; TEQ, 0.13 ( 0.07 µg/kWh; average ( 95% confidence interval of modes 3, 5, and 8 of the ECE R-49 procedure) and 2 orders of magnitude lower than emissions from a EURO-1 type 6.6.L HD engine (37) (sum of PAH, 311 ( 12 µg/kWh; TEQ recalculated from the original data, 16 ( 0.6 µg/kWh - average ( SD of two replicates with the transient US HD Federal Test Procedure). The sum of PAH adsorbed to the emitted PM was increased from 111 ( 18 to 158 ( 13 µg/g by use of emulsion, with an enrichment for the 4-6 ring compounds (data not shown) resulting in an increase in TEQ concentration on the PM from 1.1 ( 0.12 to 2.3 ( 0.14 µg/g. However, the overall effect of increasing the PAH concentrations on the particles and at the same time reducing the particle emission by use of emulsion was a slight cutback of the TEQ emissions (ng/ kWh) of 14% that, however, resulted statistically insignificant (p ) 0.22). The only other study, which addresses PAH, that the authors of the present paper are aware off is an early study of the experiences with water in gas oil diesel fuels (7), which showed that employing emulsion fuels in aged, direct
TABLE 1. Emissiona of PM, HC, CO, NOx, and B(a)P TEQ from EURO-3 LD Diesel Vehicles and HD Engine with and without Emulsions and Metal Additive Combustion Improver regulated emissions (mg/km) from LD diesel vehicles
PM HC CO NOx
fuel 1 fuel 2 fuel 1 fuel 2 fuel 1 fuel 2 fuel 1 fuel 2
regulated emissions (mg/kWh) from HD diesel
base fuel
+emulsion
fuel 4
+additive
fuel 3b
+emulsion
fuel 3
+additive
40 ( 3 35 ( 3 44 ( 7 53 ( 21 376 ( 130 371 ( 100 510 ( 180 514 ( 170
26 ( 1 21 ( 2 59 ( 7 65 ( 22 457 ( 125 446 ( 130 525 ( 180 488 ( 190
33 ( 11
28 ( 11
88 ( 2
36 ( 1
69 ( 1
70 ( 1
74 ( 53
54 ( 48
146 ( 2
167 ( 2
115 ( 2
120 ( 3
518 ( 348
416 ( 305
665 ( 7
454 ( 3
715 ( 7
745 ( 10
415 ( 150
431 ( 195
4435 ( 80
4175 ( 40
4185 ( 70
4150 ( 40
unregulated emissions (ng/km) from LD diesel
TEQ, B(a)P equiv
fuel 1 fuel 2
unregulated emissions (ng/kWh) from HD diesel
base fuel
+emulsion
base fuel
+additive
fuel 3
+emulsion
fuel 3
+additive
449 ( 230 256 ( 120
632 ( 340 305 ( 80
139 ( 10
92 ( 23
98 ( 11
84 ( 5
70 ( 9
43 ( 2
a All values indicate the fleet average ( 95% confidence interval. text for explanations.
b
This test was made with the same max power curve as for emulsion; see
TABLE 2. Emission Factors for Unregulated Individual Compounds from EURO-3 LD Vehicles and HD Engine EURO-3 LD vehiclesa
EURO-3 HD engineb
EURO-3 LD vehiclesc
particle-bound polyaromatic hydrocarbons µg/km
µg/kWh
F Phen A Fl P B(a)A Chr B(b)Fl B(k)Fl B(a)P Ind(123cd)P diB(ah)A B(ghi)Per
0.46 ( 0.13 4.5 ( 1.7 0.11 ( 0.07 1.6 ( 0.42 1.4 ( 0.41 0.10 ( 0.03 0.41 ( 0.16 0.13 ( 0.03 0.10 ( 0.04 0.11 ( 0.06 0.08 ( 0.06 0.02 ( 0.03 0.14 ( 0.06
0.11 ( 0.04 2.6 ( 0.93 0.06 ( 0.09 2.2 ( 0.50 2.7 ( 0.85 0.18 ( 0.06 0.51 ( 0.17 0.14 ( 0.03 0.009 ( 0.013 0.007 ( 0.005 0.13 ( 0.03 0.005 ( 0.005 0.14 ( 0.01
sum of PAH B(a)P-TEQ
9.1 ( 3.4 0.28 ( 0.17
8.7 ( 2.2 0.090 ( 0.017
gas-phase volatile organic compounds mg/km ethane ethene propane propene acetylene isobutane n-butane trans-2-butene 1-butene isobutene cis-2-butene propyne isopentane 1,3-butadiene
0.9 ( 0.34 11 ( 4.3 0.17 ( 0.07 5.5 ( 1.6 2.1 ( 0.79
