Saving Energy and Reducing Emissions of Both Polycyclic Aromatic

Aug 8, 2006 - Development of emulsified diesel has been driven by the need to reduce emissions from diesel engines and to save energy. Emulsification ...
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Environ. Sci. Technol. 2006, 40, 5553-5559

Saving Energy and Reducing Emissions of Both Polycyclic Aromatic Hydrocarbons and Particulate Matter by Adding Bio-Solution to Emulsified Diesel Y U A N - C H U N G L I N , * ,†,‡ W E N - J H Y L E E , * ,†,‡ C H U N - C H I C H E N , †,‡ A N D CHUNG-BANG CHEN§ Department of Environmental Engineering and Sustainable Environment Research Center, National Cheng Kung University, Tainan 70101, Taiwan, and Fuel Quality and Automobile Emission Research Division, Refining and Manufacturing Research Institute, Chinese Petroleum Corporation, Chia-Yi 60036, Taiwan

Development of emulsified diesel has been driven by the need to reduce emissions from diesel engines and to save energy. Emulsification technology and bio-solution (NOE7F) were used to produce emulsified diesel in this study. The experimental results indicated that there were no significant separation layers in W13 (13 wt % water + 87 wt % PDF), W16 (16 wt % water + 84 wt % PDF), W19 (19 wt % water + 81 wt % PDF), E13 (13 wt % NOE-7F water + 87 wt % PDF), E16 (16 wt % NOE-7F water + 83 wt % PDF), and E19 (19 wt % NOE-7F water + 81 wt % PDF) after premium diesel fuel (PDF) was emulsified for more than 30 days. In addition, there was no significant increase in damage from using these six emulsified fuels after the operation of the diesel generator for more than one year. The energy saving and reduction of particulate matter (PM) and total polycyclic aromatic hydrocarbons (PAHs) for W13, W16, W19, E13, E16 and E19, respectively, were 3.90%, 30.9%, 27.6%; 3.38%, 37.0%, 34.9%; 2.17%, 22.2%, 15.4%; 5.87%, 38.6%, 49.3%; 5.88%, 57.8%, 58.0%; and 4.75%, 31.1%, 47.3%, compared with PDF. The above results revealed that the bio-solution (NOE-7F) had a catalytic effect which elevated the combustion efficiency and decreased pollutant emissions during the combustion process. Furthermore, bio-solution (NOE-7F) can stabilize the emulsified fuels and enhance energy saving. Thus, emulsified fuels are highly suitable for use as alternative fuels. Due to the increasing price of diesel, emulsified diesel containing NOE-7F has potential for commercial application.

Introduction Alternative fuels are increasingly discussed in many countries due to environmental concerns and the increased price of fuels. The development of an alternative diesel is being driven * Address correspondence to either author. Tel: +886-6-2757575 ext. 65831; fax: +886-6-2752790; e-mail: [email protected]. † Department of Environmental Engineering, National Cheng Kung University. ‡ Sustainable Environment Research Center, National Cheng Kung University. § Chinese Petroleum Corporation. 10.1021/es061120v CCC: $33.50 Published on Web 08/08/2006

 2006 American Chemical Society

by the need to reduce the environmental impact of emissions without requiring engine modifications. Recently, water-inoil emulsions used in conventional liquid fuel engines have confirmed that water in emulsified diesel plays an important role in the combustion process (1-8). Water emulsification has the potential to slightly improve the brake efficiency and to significantly reduce the formation of thermal NO, soot, hydrocarbons, and PM in the diesel engine with no worsening of specific fuel consumption (9-10). Furthermore, emulsified diesel has also reduced the heat flux, the metal temperatures, thermal loading, and the wear-metal debris in the crankcase oil of emulsified diesel (1, 5). The combustion of emulsion layers floating on top of a water body, the micro-explosion of emulsified droplets, and the heat release of diesel emulsions in an engine can all be predicted by models (11-13). Moreover, the engine torque, powder, and brake thermal efficiency increase by increasing the water blend, while the proper brake specific fuel consumption and gas exhaust temperature decrease (7). Recently, three-phase emulsions as an alternative fuel for diesel engines have been investigated. Three-phase oil-in-water-in-oil (O/W/O) emulsions with greater water content will form a larger number of liquid droplets, leading to a faster formation rate and greater emulsion turbidity at the beginning, but a faster descending rate of emulsion turbidity afterward (6). Additives and fuel oxygenation have also been investigated in diesel combustion and emissions. An emulsion, consisting of transesterified rapeseed oil, a surfactant, and a slurry of C. vulgaris, used as a fuel in an unmodified single cylinder diesel engine, reduces NOx emission but increases the fuel consumption and carbon monoxide emission compared to base diesel (14). On the other hand, an oxygenated diglyme additive added to emulsions can increase NOx emission and combustion efficiency, decrease the fuel consumption rate and brake-specific fuel consumption, as well as the smoke opacity of PM and CO emission (15). Both emulsification activity and emulsification stability increase with the addition of an oxygenated agent, especially when the oxygenated agent is added to the water phase for oil-in-water emulsions (16). Intake oxygen enrichment and fuel oxygenation via linear structure oxygenated molecules are effective for reduction of diesel particulate matter, yielding even greater reductions in PM emissions than for fuel oxygenation via ring-structured oxygenated molecules. However, NOx emissions are greatly increased with intake oxygen enrichment, owing to either increased availability of atomic oxygen or attainment of a higher temperature during leaner combustion, which enhances the kinetics for thermal NOx formation (17). The coagulant addition significantly improved the operational time of the media filter by reducing the rate of clogging (18). The coagulant addition can also be used in emulsified diesel. For the kinds of low-emission combustion in diesel engines now being studied, emulsified diesel and putting additives in diesel have long been considered as promising approaches. Water-in-oil emulsified diesel is used in diesel engines to promote combustion efficiency. However, little research has focused on the effect of adding bio-solution to emulsified fuels. In this study, a bio-solution called natural organic enzyme-7F (NOE-7F) was added first to water and then emulsified by adding surfactants to manufacture emulsified fuels. NOE-7F is much friendlier to the environment than artificial chemical diesel additives. NOE-7F can be used to improve combustion efficiency, decrease pollutant emissions and resist, and stabilize emulsified diesel fuel in diesel generators (19). The reduction fraction of PAH emissions from diesel-engine generators by using emulsified diesel fuel VOL. 40, NO. 17, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Main Chemical Compounds of the Surfactant compound

scientific name

C13H26O2 C14H28O2 C15H30O2 C16H32O2

dodecanoic acid tridecanoic acid, 12-methyl-, methyl ester methyl tetradecanoate (1) pentadecanoic acid, 14, 12-methyl-, methyl ester (2) n-hexadecanoic acid hexadecanoic acid, methyl ester oleic acid (1) 9-octadecenoic acid, methyl ester, [E](2) 8-octadecenoic acid, methyl ester (3) 9-octadecenoic acid[Z]-, methyl ester (4) octadec-9-enoic acid (5) 9-octadecenoic acid, [E]-

C17H34O2 C18H34O2 C19H36O2

FIGURE 1. Continuous bench-scale emulsification equipment. is very significant, and has the potential for commercial applications. In this study, the emulsified diesels were made from diesel, water, surfactant, and bio-solution NOE-7F by applying emulsion technology. This study probed the measures of the stability of water-in-diesel emulsified diesel containing NOE7F. Second, the brake specific fuel consumption, energy saving, energy efficiency, PAH content in fuels, and the emission parameters of PM, soluble organic fraction (SOF), and PAH emissions from diesel-generator exhaust were compared and discussed. Finally, the input/output ratio of PAH, the feasibility of water blends as fuel for the diesel generator, and the optimum percent of water blends were assessed.

Experimental Section Production of Water-in-Diesel Emulsified Diesel. Diesel and water are inherently immiscible. A surfactant must be added to water and diesel to reduce the interfacial tension and increase the affinity among the three materials. The main chemical compounds of the surfactant used in this study are listed in Table 1. The fraction of surfactant was 1 wt % of total weight. The fraction of each compound of surfactant was 0.2 wt % ( 0.15 wt %. The water was standard city tapwater. The addition fraction of NOE-7F was 3.3 wt % of water. Waterin-diesel emulsified diesels were produced by applying the emulsion technology. The process of producing NOE-7F follows the process below. First, molasses, rice wine, acetic acid, water, and an anti-oxidant enzyme were mixed together. Second, the mixture was domesticated by adding both lactic acid bacteria and yeast. Finally, the mixture was put through a separation and purification process. The resulting product was named NOE-7F. Figure 1 shows the continuous bench-scale emulsification equipment. First, diesel, surfactant, and NOE-7F water were 5554

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pumped separately in varied fractions by weight into the mixing tank. Second, diesel, surfactant, and NOE-7F water were mixed initially by a mixer at 2100 rpm, then emulsified by a microemulsifier. Finally, varied fractions of water-indiesel emulsified diesel were produced by the emulsification equipment. Depending on residence time and motor speed, this equipment can produce between 80 and 120 L hr-1. There were seven test fuels used in this study: premium diesel fuel (PDF), W13 (13 wt % water + 87 wt % PDF), W16 (16 wt % water + 84 wt % PDF), W19 (19 wt % water + 81 wt % PDF), E13 (13 wt % NOE-7F water + 87 wt % PDF), E16 (16 wt % NOE-7F water + 83 wt % PDF), and E19 (19 wt % NOE-7F water + 81 wt % PDF). Generator and Sample Collection. The diesel generator (noncatalyst) was a QC495 with the following characteristics: four cylinders; four strokes; direct injection; watercooled; bore and stroke of 95 mm (diam.) × 105 mm; total displacement of 2976 mL; maximum torque of 169 Nm at 1950 rpm; and maximum horsepower of 40 kW at 2600 rpm. The diesel generator was made in 2003. The testing procedure was performed under steady state (75% of total torque load) for all test fuels. A sampling system was installed downstream from the diesel generator exhaust to measure smoke, suspended particles, and PAHs. PAH samples of both the particulate phase and gas phase were collected by using a PAH sampling system at a temperature below 52 °C in order to avoid desorption of PAH collected by cartridges. Particulate-phase PAHs were collected in glass-fiber filters. Before sampling, the filters were placed in an oven at 450 °C for 8 h to burn off any organic compounds that might have been present. Finally, the cleaned filters were stored in a desiccator for at least 8 h to achieve moisture equilibrium before being weighed. After the experiments, the filters were brought back to the laboratory and put in a desiccator for 8 more h to remove the moisture. They were then weighed again to determine the net mass of particles collected. Gas-phase PAHs were collected in a three-stage glass cartridge containing a polyurethane foam (PUF) plug followed by XAD-16 resin. The glass cartridge was packed with 5.0 cm of XAD-16 resin sandwiched between a 2.5-cm upper PUF plug and a 2.5-cm bottom PUF plug. Silicone glue was used to seal and hold these two pieces of PUF to prevent resin from leaking out during the sampling and extraction processes. After 8 h of adherence, the new PUF/ resin cartridge was cleaned up by Soxhlet extracting for 1 day each with distilled water, methanol, dichloromethane, and finally n-hexane for a total of 4 days and then these PUF/ resin cartridges were placed in a vacuum oven at 60 °C for 2 h to dry and evaporate the residual solvent in them. After drying, each PUF/resin cartridge was individually wrapped in hexane-washed aluminum foil and stored in a refrigerator at 4 °C and transported in clean screw-capped jars with Teflon cap liners before sampling. Each glass fiber filter was transported to and from the field in a glass box, which was also wrapped with aluminum foil. In this study, each of the threestage PUF/resin glass cartridges was analyzed. We found that less than 5% of total gas-phased PAHs (i.e., stage 1 + stage 2 + stage 3) were collected by stage 3, suggesting the breaking through of gas-phase PAHs was negligible. PAH Analysis. Each collected sample (including particulate and gaseous PAH samples) was extracted in a Soxhlet extractor with a mixed solvent (n-hexane and dichloromethane; vol/vol, 1:1; 500 mL each) for 24 h. The extract was then concentrated with nitrogen (N2), cleaned up with sodium sulfate, and reconcentrated to exactly 1.0 mL by N2. The analysis method of PAHs has been described in our previous studies (20-26). The PAH contents were determined with a Hewlett-Packard (HP) gas chromatograph (GC) (HP 5890A; Hewlett-Packard, Wilmington, DE), a mass selective detector (MSD) (HP 5972), and a computer workstation (Aspire C500;

TABLE 2. PAH Homologues number of rings 2 3

4

5

6 7

PAH

abbreviation

low-molecular-weight PAHs (LMW-PAHs) naphthalene Nap acenaphthylene acenaphthene fluorene phenanthrene anthracene

AcPy Acp Flu PA Ant

middle-molecular-weight PAHs (MMW0PAHs) fluoranthene FL pyrene Pyr benz[a]anthracene BaA chrysene CHR high-molecular-weight PAHs (HMW-PAHs) cyclopenta[c,d]pyrene CYC benzo[b]fluoranthene benzo[k]fluoranthene benzo[e]pyrene benzo[a]pyrene perylene dibenz[a,h]anthracene benzo[b]chrysene indeno[1,2,3-cd]pyrene benzo[ghi]perylene coronene

BbF BkF BeP BaP PER DBA BbC IND BghiP COR

Acer, Taipei, Taiwan). This GC/MSD was equipped with a capillary column (HP Ultra 2, 50 m × 0.32 mm × 0.17 µm) and an automatic sampler (HP-7673A) and operated under the following conditions: injection volume of 1 µL; splitless injection at 310 °C; ion source temperature at 310 °C; oven temperature from 50 to 100 °C at 20 °C/min, 100 to 290 °C at 3 °C min-1, and held at 290 °C for 40 min. The masses of primary and secondary PAHs ions were determined by using the scan mode for pure PAH standards. The PAHs were qualified by using the selected ion monitoring (SIM) mode. PAH homologues are listed in Table 2. The total-PAH data for the HDDE exhaust is the summation of 21 individual PAHs. The GC/MSD was calibrated with a diluted standard solution of 16 PAH compounds (PAH mixture-610M; Supelco, Bellefonte, PA) plus five additional individual PAHs obtained from Merck (Darmstadt, Germany). Analysis of serial dilutions of PAHs standards showed the detection limit (DL) for GC/ MSD was between 28 and 324 pg for the 21 PAH compounds. Ten consecutive injections of a PAH 610-M standard yielded an average relative standard deviation of GC/MSD integration area of 6.96%, within a range of 4.33-9.76%. The R 2 of calibration in 21 PAH compounds ranged from 0.9945 (CYC) to 0.9993 (PA). Following the same experimental procedures used for sample treatment, recovery efficiencies were determined by processing a solution containing known PAH concentrations. This study showed the recovery efficiencies for the 21 PAH compounds ranged from 0.881 to 0.952, with an average value of 0.918. Analyses of field blanks, including aluminum foil, glass-fiber filters, and PUF/XAD-16 cartridges, revealed no significant contamination (GC/MSD integrated area < detection limit). Data Analysis. The total PAH concentration was the sum of the concentrations for the 21 PAH compounds in each collected sample. To assess PAH homologues distribution for each collected sample, total PAHs were further classified into three categories of the LM-PAHs, MM-PAHs, and HM-

PAHs. Moreover, considering that several PAH compounds are known human carcinogens, the carcinogenic potencies of PAH emissions from each emission source were also determined. In principle, the carcinogenic potency of a given PAH compound is assessed on the basis of its benzo[a]pyrene equivalent concentration (BaPeq). Calculation of the BaPeq concentration for a given PAH compound uses its toxic equivalent factor (TEF), which represents the relative carcinogenic potency of the given PAH compound, using benzo[a]pyrene as a reference compound to adjust its original concentration. Among the list of TEFs available (27-30), the one by Nisbet and LaGoy (29) has been demonstrated to best reflect the actual state of knowledge of the toxic potency of each individual PAH species. Based on this TEF, the carcinogenic potency of total PAHs (total BaPeq) can be assessed by the sum of BaPeq concentrations estimated for each PAH compound with a TEF.

Results and Discussion Characterization of Emulsified Diesel. The diameters of oil particles in each of the tested fuels were in the following ranges: W13 50-300 nm, W16 50-400 nm, W19 100-400 nm, E13 50-300 nm, E16 50-200 nm, and E19 100-400 nm. The volumetric proportions of emulsification layer water-in-oil (W/O) emulsions with various percentages of water blends were kept motionless for 120 days. Results show that the emulsions (W13, W16, W19, E13, E16 and E19) did not separate when kept motionless for 30 days. When emulsified diesels were fueled on the diesel generator, about 80% was heated and overflowed and went back to the storage tank. After 120 days, less than 1 v/v% of the diesel-in-water layer was found at the bottom. These results indicated that the stability of emulsified diesel is suitable for use as an alternative fuel. Brake Specific Fuel Consumption, Energy Saving, and Energy Efficiency. The brake specific fuel consumption (BSFC) values in PDF, W13, W16, W19, E13, E16, and E19 were 0.361, 0.390, 0.404, 0.422, 0.382, 0.394, and 0.411 L kWh-1, respectively. The BSFC increased with increasing water blends by considering diesel + water as total fuel. The mean increasing fraction of BSFC was 8.30%, 11.9%, 16.9%, 5.82%, 9.02%, and 13.8% for W13, W16, W19, E13, E16 and E19, respectively, compared with PDF. This was because the gross heat value of water equals zero. Hence, to maintain the same electric power, the BSFC of the emulsified diesel increased with increasing water blends. Similar results for the specific fuel consumption of the water blends were also found. Scragg et al. (14) found that the mean increasing fraction of fuel consumption was 35.9% for biodiesel algal emulsion. AbuZaid (7) found that the BSFC decreased as engine speed increased, reached a minimum and then rose at high speeds. At low speeds, the heat lost through the combustion chamber walls was proportionately greater and combustion efficiency was poorer, resulting in higher fuel consumption for the power produced. At high speeds, the fraction power increased at a rapid rate, resulting in a slower increase in the power than in fuel consumption with a consequent increase in BSFC (7). Furthermore, the energy saving (denoted as ES) varied with the fraction of water blends in neat diesel and were 3.90%, 3.38%, 2.17%, 5.87%, 5.88%, and 4.75% for W13, W16, W19, E13, E16, and E19, respectively, compared with PDF (Figure 2A). It was found that NOE-7F increased combustion efficiency when compared with W13 and E13. Additionally, ES rose as NOE-7F water increased, and ES reached a maximum and then decreased at high water blends. The energy efficiency (denoted as EE) was defined as output energy divided by input energy. The EEs of PDF, W13, W16, W19, E13, E16, and E19 were 27.42%, 28.35%, 28.17%, 27.94%, 28.88%, 28.92%, and 28.48%, respectively. Thus, the average VOL. 40, NO. 17, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Mean PAH Content in the Base Diesel Fuel (mg L-1, n ) 7) PDF number of rings

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TEFa

LMW-PAHs 9,730-12,700 10,900 8,570-12,100 10,400 5,380-8,410 6,760 3,260-5,970 4,620 1,300-1,930 1,560 867-1,850 1,330

21.4 23.2 20.4 24.1 19.4 23.7

0.001 0.001 0.001 0.001 0.001 0.01

4

FL Pyr BaA CHR

MMW-PAHs 468-986 86.7-143 66.7-99.2 40.7-6.17

734 118 75.8 51.4

20.2 25.3 26.8 22.7

0.001 0.001 0.1 0.01

5

CYC BbF BkF BeP BaP PER DBA BbC IND BghiP COR

HMW-PAHs 20.8-40.7 95.1-168 16.9-37.2 6.34-15.2 148-236 6.57-15.9 73.5-167 69.8-142 64.3-123 40.2-63.5 7.57-15.4

30.8 132 19.5 10.3 208 10.7 114 98.9 90.8 50.8 11.6

24.2 22.9 20.5 25.0 26.8 27.1 29.4 27.1 28.6 21.5 24.8

b 0.1 0.1 b 1.0 b 1.0 b 0.1 0.01 b

7

FIGURE 3. Emission factor of PM and SOF varied with the fraction of water blends.

RSD (%)

Nap AcPy Acp Flu PA Ant

ΣLMW-PAHs ΣMMW-PAHs ΣHMW-PAHs total PAHs total BaPeq a

mean

range

2 3

6

FIGURE 2. Energy saving (ES) and energy efficiency (EE) varied with the fraction of water blends in neat diesel.

PAH

Ref 29.

b

31,100-41,200 735-1,120 682-965 29,400-44,200 327-502

35,600 979 777 37,300 403

20.3 23.9 20.8 21.0 20.8

No TEF has been suggested.

increase in EE for E16 was approximately 5.45% compared with PDF (Figure 2B). This was because when the temperature of water-in-diesel emulsified diesel in the cylinder went up to 105 °C, the water droplets evaporated rapidly and then broke fuel droplets. Smaller fuel droplets were produced due to a burst of evaporation. The surface area of the fuel droplets increased several times. This caused the air and fuel droplets to mix well and caused complete combustion. Thus, the particulate became smaller due to the combustion of a second nebulization of water-in-diesel emulsified diesel in the cylinder, making it easier for the particulates to mingle with water. These hot particulates reacted with water and produced flammable gas, CO, and H2. NOE-7F can be used for resisting and stabilizing emulsified diesel fuel in the diesel generator. Even though water partially consumed the latent heat, the loss of latent heat was smaller than the heat released from the combustion of flammable gas. However, the amount of water added to the emulsified diesel was restricted. Adding too much water is not beneficial for combustion because the partial loss of latent heat become too large for the fuel to burn completely. Scragg et al. found that the mean increasing fraction of net efficiency was 3.90% for biodiesel algal emulsion (14). Abu-Zaid found that the average increase in brake thermal efficiency for 20% water emulsion was approximately 3.5% over the use of diesel for various engine speeds ranging from 1200 to 3300 rpm (7). These results revealed that emulsified diesel containing NOE-7F, initially regarded only as a bio-solution, can also improve combustion efficiency and save energy when used in diesel engines. Emission of PM and SOF in the Exhaust of the Diesel Generator. Particulate matter from an engine includes three main components: soot formed during combustion, heavy

TABLE 4. Mean PAH Concentration in the Exhaust of the Diesel Generator (ng m-3) number of rings

2 3

Nap AcPy Acp Flu PA Ant

4

FL Pyr BaA CHR

5

CYC BbF BkF BeP BaP PER DBA BbC IND BghiP COR

6 7

ΣLMW-PAHs ΣMMW-PAHs ΣHMW-PAHs total PAHs total BaPeq a

Ref 29.

PDF (n ) 3)

PAH

b

2,040 11.0 322 1,110 3,310 26.0

W13 (n ) 3)

W19 (n ) 3)

E13 (n ) 3)

1,530 8.25 239 832 2,500 19.8

LMW-PAHs 1,450 1,880 6.95 9.27 199 253 701 945 2,240 2,920 15.5 19.5

1,080 7.12 176 596 1,710 15.6

229 136 4.59 11.5

MMW-PAHs 185 241 115 150 3.63 4.58 9.03 12.2

146 91.0 3.02 6.54

312 185 6.21 15.0 4.67 3.51 4.06 16.7 1.36 1.14 0.370 18.7 2.06 2.58 14.9 6,820 518 70.1 7,410 11.0

W16 (n ) 3)

HMW-PAHs 2.76 3.43 2.01 2.69 2.44 3.22 7.91 10.4 0.774 1.05 0.707 0.917 0.187 0.240 11.4 14.1 1.48 1.94 1.37 1.94 7.46 9.86

3.52 2.65 3.02 12.7 1.03 0.844 0.277 13.9 1.55 1.92 11.0 5,130 381 52.4 5,560 8.30

4,610 313 38.5 4,970 7.07

6,030 407 49.8 6,480 9.26

2.58 1.97 2.36 6.44 0.392 0.656 0.223 10.8 0.686 1.03 7.31 3,590 246 34.4 3,870 5.46

E16 (n ) 3)

725 10.2 217 619 1,400 11.1 134 69.7 2.32 5.67 2.40 1.96 2.12 4.69 0.320 0.684 0.280 8.56 0.439 1.03 5.74 2,980 211 28.2 3,220 4.63

E19 (n ) 3)

TEFa

920 9.09 258 790 1,810 12.2

0.001 0.001 0.001 0.001 0.001 0.01

176 67.6 2.39 6.15

0.001 0.001 0.1 0.01

2.12 2.34 2.47 2.99 0.502 0.941 0.278 5.16 0.173 0.274 3.61

b 0.1 0.1 b 1.0 b 1.0 b 0.1 0.01 b

3,800 252 20.9 4,070 5.73

No TEF has been suggested.

TABLE 5. Mean PAH Emission Factors in the Exhaust of the Diesel Generator (µg kWh-1, µg L-1)a PDF W13 W16 W19 E13 E16 E19 (n ) 3) (n ) 3) (n ) 3) (n ) 3) (n ) 3) (n ) 3) (n ) 3) total PAHs µg kWh-1 µg L-1 total BaPeq µg kWh-1 µg L-1

410 297 1,140 762

267 660

347 822

208 543

172 436

216 527

0.606 0.443 0.380 0.496 0.293 0.247 0.305 1.68 1.14 0.941 1.12 0.766 0.628 0.742

a PDF, premium diesel fuel; W13, 13 wt % water + 87 wt % PDF; W16, 16 wt % water + 84 wt % PDF; W19, 19 wt % water + 81 wt % PDF; E13, 13 wt % NOE-7F water + 87 wt % PDF; E16, 16 wt % NOE-7F water + 83 wt % PDF; E19, 19 wt % NOE-7F water + 81 wt % PDF.

hydrocarbon condensed or absorbed on the soot, and sulfates. The emission factor of PM in the exhaust of the diesel generator fueled with emulsified diesel is shown in Figure 3A and B. The emissions of PM in W13, W16, W19, E13, E16, and E19 were all smaller than that with PDF. The mean reduction fractions of PM emission factors (PDF ) 83.7 mg kWh-1, 232 mg L-1) from the exhaust of the diesel generator for W13, W16, W19, E13, E16, and E19, respectively, compared with PDF were 30.9%, 36.2%; 37.0%, 44.0%; 22.2%, 33.6%; 38.6%, 41.8%; 57.8%, 61.4%; and 31.1%, 39.7%. These data indicated a strong potential for reductions in PM emissions from current diesel engines by optimizing the fuel composition. In engine exhaust, the soluble organic fraction (SOF) is derived partly from the lubrication oil, partly from unburned fuel, and partly from compounds during combustion. The SOF values were 46.4%, 31.7%, 29.3%, 34.5%, 30.5%, 26.9%, and 31.6% for PDF, W13, W16, W19, E13, E16, and E19,

respectively (Figure 3C). The SOF value tends to be similar to the PM emissions from the diesel-generator exhaust. Comparing the SOF data of W16 (29.3%) and W19 (34.5%) with that of E16 (26.9%) and W19 (31.6%), it can be seen that when the water fraction was more than 16% in the emulsified fuels, increasingly incomplete combustion of the emulsified diesel occurred, and resulted in an increase of PM and SOF in the diesel generator exhaust. PAH Emissions from the Diesel-Generator Exhaust. PAHs originally existed in the fuel. Table 3 shows the mean total PAH content in base diesel fuel was 37 300 µg L-1 (range: 29 400-44 200 µg L-1). The mean LMW PAHs content was 35 600 µg L-1, which accounted for 95.4% of the mean total-PAH mass in the base diesel fuel. The mean contents of BaP and DBA, the most carcinogenic PAHs, were 208 and 114 µg L-1, respectively. BaP content was the highest mean content among ten HMW PAHs. The mean content of total BaPeq was 403 µg L-1, which accounted for 1.08% of mean total-PAH mass in the base diesel fuel. These results indicated that the PAHs in the base diesel fuel were primarily dominant in the LMW PAHs. PAH concentration in the diesel-generator exhaust is listed in Table 4. Total PAH emission concentration in the dieselgenerator exhaust decreased with increasing water blends, but increased after 16% water blend. The mean reduction fractions of total PAH concentration from the diesel-generator exhaust were 25.0%, 32.9%, 12.6%, 47.8%, 56.5%, and 45.1% for W13, W16, W19, E13, E16, and E19, respectively, compared with PDF (7410 ng m-3). LMW PAH concentrations in test fuels were all higher than MMW and HMW PAH. This result corresponded to previous experimental results: PAH content was primarily dominant in LMW PAHs. Total BaPeq emission concentration in the diesel-generator exhaust followed a similar tendency. The mean reduction fractions VOL. 40, NO. 17, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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difficulty of predicting the combustion products of PAHs. The O/I ratio would be a useful tool to explain the fate of PAHs during the combustion process in a diesel generator. Of the seven test fuels, PDF, W13, W16, W19, E13, E16, and E19, the O/I ratios of most individual PAHs were lower than 1.0. These results represented that the majority of PAH mass was destroyed during the combustion process in diesel generators fueled with emulsified diesel. The O/I ratio of total PAHs in PDF was 2.94%, or 1.28, 1.44, 1.12, 1.80, 2.17, and 1.74 times higher magnitude than in W13, W16, W19, E13, E16, and E19, respectively. The O/I ratio of total BaPeq in PDF was 0.406%, or 1.28, 1.50, 1.17, 1.90, 2.25, and 1.85 times higher magnitude than in W13, W16, W19, E13, E16, and E19, respectively (Figure 4). The above results indicated that emulsified fuels have higher depletion efficiency than PDF. NOE-7F in particular had a catalytic effect which elevated combustion efficiency and destroyed pollutants such as PAHs in the diesel engine.

Acknowledgments This research was supported in part by National Science Council in Taiwan under grant number NSC-93-2211-E-006038. The authors also gratefully acknowledge the contributions of Mr. Hsiao-Chung Hou and Miss UnSam Ha, Department of Environmental Engineering, National Cheng Kung University, for helping with the laboratory work.

Literature Cited FIGURE 4. Output/input (O/I) ratio of PAHs for the diesel generator varied with the fraction of water blends: (a) total PAH (b) total BaPeq. were 24.8%, 36.0%, 16.2%, 50.6%, 58.1%, and 48.1% for W13, W16, W19, E13, E16, and E19, respectively, compared with PDF (11.0 ng m-3). PAH emission factors (µg L-1 or µg kWh-1) on both total PAHs and total BaPeq (denoted EFtotal-PAH and EFBaPeq, respectively) were calculated and are listed in Table 5. As shown in the sequence for magnitudes of EFtotal-PAH and EFBaPeq, analysis identified the mean reduction fraction of total PAHs emission factor (PDF ) 410 µg kWh-1) from the diesel-generator exhaust to be 27.6%, 34.9%, 15.4%, 49.3%, 58.0%, and 47.3% for W13, W16, W19, E13, E16, and E19, respectively, compared with PDF. As for EFBaPeq in µg kWh-1, the mean reduction fractions of total BaPeq (PDF ) 0.606 µg kWh-1) from the diesel-generator exhaust were 26.9%, 37.3%, 18.2%, 51.7%, 59.2%, and 49.7% for W13, W16, W19, E13, E16, and E19, respectively, compared with PDF. A similar tendency in the mean reduction fraction of EFtotal-PAH and EFBaPeq in µg L-1 can be seen. This was because the PAH content of water, NOE-7F, and the surfactant were close to zero. However, incomplete combustion occurred and resulted in an increase of PAH emissions in water fractions as high as 16%. Farfaletti et al. found that the mean reduction fractions of total BaPeq emission from the exhaust of the heavyduty engine were 14% and 39% for emulsions with up to 20% water and the Cerium-based additive, respectively (31). These results revealed that adding NOE-7F water to base diesel and forming emulsified fuels with water blends lower than 16% is a good way to decrease PAH emissions from the exhaust of diesel generators. Output/Input Mass (O/I) Ratio of PAHs for the Diesel Generator. An output/input mass (O/I) ratio greater than 1.0 implies that PAH is generated during the combustion process. On the other hand, an O/I ratio less than 1.0 means that PAH is depleted during the combustion process, due to the complexity of PAH formation mechanisms and the 5558

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Received for review May 11, 2006. Accepted June 30, 2006. ES061120V

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