Reduction in emissions of nitrogen oxides, particulate matter, and polycyclic aromatic hydrocarbon by adding water-containing butanol into a diesel-fueled engine generator

Fuel 93 (2012) 364–372

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Reduction in emissions of nitrogen oxides, particulate matter, and polycyclic aromatic hydrocarbon by adding water-containing butanol into a diesel-fueled engine generator Sheng-Lun Lin a,b,⇑, Wen-Jhy Lee a,b,⇑, Chia-fon F. Lee c,d, Yo-ping Wu e a

Department of Environmental Engineering, National Cheng Kung University, Tainan City 70101, Taiwan, ROC Sustainable Environment Research Center, National Cheng Kung University, Tainan City 70101, Taiwan, ROC Center for Combustion Energy and State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China d Department of Mechanical Science and Engineering, University of Illinois at Urbana – Champaign, IL 61801, USA e Department of Chemical Engineering and Materials Engineering, National Ilan University, Ilan City 26047, Taiwan, ROC b c

a r t i c l e

i n f o

Article history: Received 7 October 2010 Received in revised form 15 November 2011 Accepted 17 November 2011 Available online 3 December 2011 Keywords: Butanol Biodiesel NOx PAHs Carbon dioxide

a b s t r a c t Three groups of n-butanol–diesel blends with 0, 0.5, and 1.0 wt.% water-content were investigated to simulate the hydrated butanol produced by acetone–butanol–ethanol fermentation and a simple distillation treatment. Both 30-day standing and centrifugal test results showed that 15 wt.% n-butanol (BT) was the minimum additive ratio to stabilize the 1.0 wt.% water content diesel blend, while those blends that contained 0 or 0.5 wt.% water could remain as stable one-phase clear liquids by adding just 5 wt.% BT. These stable diesel blends were further examined in a heavy-duty diesel-fueled engine generator (HDDEG). Using BT-diesel blends increased the indicated specific fuel consumption (ISFC) because of the lower heating value of n-butanol, while the micro-explosions that occurred could reduce the ISFC when using 0.5 wt.% water-containing BT-diesel blends. NOx emissions increased with the increasing BT content at a low additive ratio (5–15 wt.%), and reduced when adding a higher amount of BT (>15 wt.%). PM, total-PAHs, and total-BaPeq emissions were all significantly reduced when the increasing BT additive ratio contained either 0, 0.5, or 1.0 wt.% water because of the lower sulfur and higher oxygen fuel contents. On the other hand, the CO emission level went up with the addition of BT. Notably, the diesel blends with 0.5 wt.% water only slightly increased ISFC when low fractions of BT were added, i.e. 0.40% and 0.81% ISFC increases with the addition of 5 and 10 wt.% BT, respectively. In addition, there were significantly lower NOx, PM, Total-PAHs and Total-BaPeq, emissions with the blends than with regular diesel. With the aim of achieving both good energy performance and less pollutant emissions, the 5 and 10 wt.% BT additive with 0.5% water content blends were the most suitable for practical use in an HDDEG without any engine modifications or changes in controls. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Since Diesel invented the first compression-ignition engine in 1892, both researchers and manufacturers have worked to make such engines operate with greater thermal efficiency, higher output power, and lower emissions of carbon monoxide (CO) and unburned hydrocarbon (HC). However, the sharply increasing price of crude oil and dwindling reserves over the last few decades [1] show the dangers of over-dependence on fossil fuels. Additionally, the emissions of particulate matter (PM), sulfur dioxide (SO2), ⇑ Corresponding authors. Address: Department of Environmental Engineering, National Cheng Kung University, Tainan City 70101, Taiwan, ROC. Tel.: +886 6 2757 575x65831; fax: +886 6 275 2790 (S-L. Lin). E-mail addresses: [email protected] (S.-L. Lin), [email protected] (W.-J. Lee). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.11.042

nitrogen oxides (NOx) and polycyclic aromatic carbons (PAHs) released when burning such fuels cause environmental and public health problems [2–4], as well as being associated with global warming due to the influence of greenhouse gas (GHG) emissions, especially carbon dioxide (CO2). Therefore, developing cleaner and renewable energy sources has become a pressing concern. Biodiesel and bioethanol are the two most popular renewable and oxygenated fuels currently in use. As extracting fuel from crops is more environmentally friendly and CO2 conservative than petroleum processing, several processes have been developed to transform neat vegetable oil into biodiesel with similar properties to those of a regular diesel. The transesterification process is the most practical one, with high production yields of around 80–95% and the ability to significantly decrease the viscosity of neat vegetable oil [5]. In a study on biodiesel emissions, Scholl and Sorenson [6] reported that CO, unburned HC, NOx, and smoke

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emissions were reduced by utilizing a soybean methyl ester (SME) in a CI engine, while a 15.1% increase in energy efficiency, 3.55% and 7.49% reductions in PM10 and PM2.5, respectively, and 43.0–90.2% and 63.1–89.6% decreases in total-PAH and total-BaPeq emissions, respectively, were found by using palm-biodiesel and paraffinic-palm-biodiesel blends in Lin et al. [7–9]. However, land competition with food crops, pretreatment cost, and secondary pollutants associated with the transesterification process have limited the use of crop-based biodiesels. Bioethanol has a high octane number and has been investigated for its role in enhancing the combustion reaction and reducing CO, HC, and PM emissions in gasoline engines [10]. Although pure ethanol with a lower cetane index is considered to be less suitable for use in a compression-ignition (CI) diesel engine, low fraction replacements have also been investigated. For example, Sayin [11] blended 5% and 10% dehydrated ethanol with a regular diesel, and tested it in a direct injection diesel engine. Although the use of ethanol–diesel blends showed reductions in smoke opacity and emissions of CO and HC, the brake specific fuel consumption (ISFC) increased because the heating value of ethanol (28.40 MJ kg1) is significantly obviously lower than that of a regular diesel (42.74 MJ kg1). In addition, 99.5% pure ethanol was required to form a stable ethanol–diesel blend, while the simple distillation process could only concentrate it to 95.6% [12,13]. Martinez-Frias estimated that the ethanol dehydration process would cost 14% in net energy balance over the whole bioethanol production process [14]. Dieselhol is an oxygenated fuel composed of biodiesel, ethanol, and diesel, in which the low viscosity of the alcohol can balance the high viscosity of the biodiesel. Furthermore, the low cetane index and heating value of the ethanol can also be increased by blending with biodiesel, which has a high cetane index and heating value. Shi et al. showed that a blend of 80% or 85% diesel with 20% or 15% BE (methanol soyate contains 20 wt.% ethanol) has significantly lower PM and CO emissions, but higher NOx emission [15]. Kwanchareon et al. obtained similar results, which showed that the emissions of CO and HC were obviously reduced, whereas the NOx emission was increased by using a blend of 80% diesel, 15% biodiesel, and 5% ethanol at high engine load [16]. However, the high production cost of crop-based dehydrated ethanol and biodiesel hinder the practical use of dieselhol in diesel engines. Gnansounou et al. presented a lifecycle assessment of wheat-to-bioethanol, with the outcome affected by the method used to allocate the impacts between the co-products, the type of reference systems and the choice of functional unit. They found that the GHG emission equivalents increased from 0.047 kg CO2eq km1 (i.e. grassland to cultivated land) to 0.249 kg CO2eq km1 (i.e. forested land to cultivated land), that is 80% to +5% with respect to gasoline, respectively [17]. Additionally, the water consumption when producing energy crop-based ethanol (e.g. corn ethanol, 10–324 L water L1 ethanol) was reported to be greater than that of both cellulose-based ethanol (Switchgrass ethanol, 1.9–9.8 L water L1 ethanol) and gasoline (US conventional crude, 3.4–6.6 L water L1 ethanol) by Wu et al. [18]. However, the problems that crop-based biofuels face with regard to land-use and water demand have hindered their widespread adoption. Butanol is another renewable biofuel that is now attracting more attention, since it is less hydrophilic, has a higher heating value and cetane index, lower vapor pressure, and greater miscibility than ethanol, which make it more suitable for blending with conventional gasoline or diesel fuel [19]. Szwaja et al. indicated that the n-butanol can directly substitute for gasoline either as a neat fuel or in a blended form from a combustion and energy density perspective for spark-ignition (SI) engines, because of its similar thermo-physical properties to those of conventional gasoline

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[20]. The combustion modeling of pure bio-butanol demonstrates that a higher oxygen content shortens the ignition delay [21,22]. In addition, Rakopoulos et al. indicated that the emissions of soot, NOx, CO were reduced while those of ISFC and HC increased with the increasing addition of pure butanol (8%, 16%, and 24%) in both a Hydra diesel engine and a bus engine [19,23]. Yao et al. also found similar results, that soot and CO emissions were reduced by using n-butanol/diesel blends with multi-injections, while the amount of oxygen did not significantly affect the NOx emissions [24]. Acetone–butanol–ethanol (ABE) fermentation is a developing technology that transforms molasses, whey permeate, wheat straw, corn stover, corn fiber, and other traditional agricultural wastes into biofuels (e.g. acetone, butanol, and ethanol). Table 1 shows the recovery rates and fractions of three products of ABE fermentation obtained in several studies. Generally, the ratio of ABE used has been 3:6:1, with butanol as the major product (55.7–73.9 wt.%) in ABE solution [25–30]. Additionally, a byproduct of biodiesel production, glycerol, has been used to produce significant yields of mainly 1, 3-propanediol and butanol [31–33]. Butanol production are attracting more attention, because it is relatively more valuable as either a solvent or a fuel. However, the techniques for separating butanol from ABE products, especially dehydration, require both extra cost and energy consumption. A comprehensive search of the literature shows that are relatively few studies that examine hydrated butanol–diesel blends. Therefore, the current study fixed 0.5 or 1.0 wt.% water content and varied the butanol additive fractions from 5 to 25 wt.% to simulate the 2–25 wt.% (0.5/25–5/20) water content in hydrated butanol and when blended with the regular diesel. The fuel stabilities, engine performances, and pollutant emissions of these mixtures were also examined in the current study.

2. Experimental section 2.1. Fuel preparations and stability tests A premium diesel fuel (D100) produced by Chinese Petroleum Corporation (CPC) was used as a base fuel in current study. Analytical-grade n-butanol (BT) obtained from J.T. Baker (>99.5% purity) was utilized to simulate the bio-butanol produced by ABE fermentation. Additionally, to simulate the dehydrated butanol treated by a simple distillation separation process after ABE fermentation, the water contents were fixed at 0.5 and 1.0 wt.%, while the butanol additive fractions were varied from 5 to 10, 15, 20, and 25 wt.%. 2–17 wt.% (0.5/25.5–1/6) water-containing butanol blends were investigated in the current study. The hydrated butanol–diesel blends were prepared by adding the specific ratios of water into the pure n-butanol, and then blending the hydrated butanol with a conventional diesel by employing a lab-scale 1-L mixer with 3000 rpm for 15 min. The premixed products were then homogenized in an ultrasonic tank at 40 kHz and 120 W ultrasonic power output for 15 min. The products were then tested for their stability. The stability of the diesel blends was examined by two gravitational and centrifugal tests. In the gravitational test, the appearance of each diesel blend was recorded after standing for 30 days at 25 °C to determine the one-month-storage stability. Furthermore, a centrifugal stability test was conducted at 5000 rpm for 15 min to predict the stability changes of diesel blends after longer term storage. After the two stability tests, the four levels of the phase appearances of the blended fuels formed an empirical index of stabilities: (a) one-phase clear liquid; (b) one-phase liquid crystalline; (c) two-phase liquid crystalline; (d) two-phase clear liquid. The order of stability is (a) > (b) > (c) > (d), so that an unstable blend will transform its appearance from (a) to (d) over time.

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Table 1 Recovery rates and compositions of acetone butanol ethanol solution using C. acetobutylicum and/or C. beijerinckii in ABE fermentation. References

Qureshi et al. [25] Ezeji et al. [26] Ezeji et al. [27] Ezeji et al. [28] Qureshia et al. [29] Gu et al. [30]

Acetone

Butanol

Ethanol

1

1

1

gL

wt.%

gL

3.32–6.12 5.3–27.4 5.20 4.0–24.1 3.74 4.60

25.3–43.6 29.2–36.1 29.5 22.0–32.2 31.3 23.0

8.24–12.8 11.9–46.4 11.9 13.4–56.2 7.40 14.1

Furthermore, a greater ratio of the separate layer in a two-phase clear liquid means the diesel blend is less stable. On the other hand, a diesel fuel in a ‘‘one-phase clear’’ condition is relatively stable, and can be used in a diesel-fueled engine generator. 2.2. Diesel-fueled engine generator test The brake specific fuel consumption (ISFC) and pollutant emissions were examined in a diesel-fueled engine generator. A fourstroke, single horizontal cylinder engine made by Yanmar S. P. Co. was employed in this system (Table 2). A direct injection system with 196.2 bar (2845 psi) injection pressure and injecting at BTDC 17.0 CA was employed in the current study, and there was no modification to the injection. More details of the whole testing system can be found in Lin et al. [34], while the generator load was controlled by a varied resistance with 100 parallel 50 W bulbs that was equal to maximum 5 kW. The generator loads were fixed at 3.2 kW, 80% of the maximum load of the engine output power (4 kW), because this load is commonly used for a continuous electricity generation process. After the generator load was set, the engine rotational speed was tuned to fit the 110 V output voltage, which is the regulated standard in Taiwan. Additionally, the volume of fuel consumed in the sampling period was measured using a one liter graduated cylinder inlet unit. The indicated fuel specific consumption (ISFC) of a generator was then calculated by the ratios of the fuel consumption rate (mL h1) and electricity output power (kW), as shown in Eq. (1). The indicated efficiency (IE,%) was also calculated using Eq. (2) to analyze the fuel efficiency without the effect of different heating values. 1

ISFC ¼ V=Pt ðmL kW

1

h Þ

IE ¼ Pt=VH ð%Þ

ð1Þ ð2Þ

where V represents the fuel consumption (mL) during each sampling time, P stand for the power generated (kW) by the dieselfueled engine generator with 110 V, t represents the sampling time (h) and H stands for the heating value of each diesel blend.

Table 2 Specifications of the tested diesel fueled engine. Specifications Manufacturer Type Bore  Stroke Displacement volume Rated engine speed Output power Torque Ignition system Injection type Injection timing Injection pressure Lubrication system Cooling system

YANMAR S. P. Co., Ltd., Thailand Four-stroke, single horizontal cylinder engine 88  96 mm 583 c.c. 2400 rpm 4.0 kW 1.63 kg-m Compression ignition Direct injection BTDC 17.0 196.2 bar (2845 psi) Forced-feed type Water-cooled

wt.%

gL

55.7–73.9 61.1–69.5 67.6 63.2–73.6 62.0 70.5

0.00–0.51 0.3–2.1 0.50 0.6–1.1 0.79 1.30

Total ABE wt.%

g L1

wt.%

0.00–2.74 1.27–2.82 2.80 1.23–4.60 6.62 6.50

12.2–19.2 17.7–75.9 17.6 18.2–81.3 11.9 20.0

100 100 100 100 100 100

The gaseous pollutant emissions in the tail pipe exhaust were monitored online using a portable gaseous pollutants analyzer (IMR2800). Specifically, nitrogen oxides (NOx) were measured with an electro-chemical detector (ECD) in the ranges of 0–5000 and 0–4000 ppm V, while carbon monoxide (CO) was determined using a non-dispersive infrared (NDIR) analyzer in the ranges of 0–10 and 0–20 wt.%. A vacuum pump provided a constant flow rate to collect the particulate matter (PM) on a silicon glass fiber filter. The PM mass on each filter was determined using an electronic analytical microbalance (Sartorius ME 5-F) with 0.01 mg accuracy. 2.3. PAH analysis In the current study, all sampling and analytical processes of PAHs followed the A730.70C method proposed by the National Institute of Environmental Analysis (NIEA) in Taiwan. Particulate-phase PAHs were collected on a glass fiber filter, which was pretreated at 450 °C for 8 h in an oven to remove all organic compounds. The net mass of particles was then determined by subtracting the mass of the initial filter from the final one. Gas-phase PAHs were collected in three-stage glass cartridges in preliminary sampling work. Since the masses of 21 individual PAHs in the third stage were 0.1–2.6% of total three stages, the two-stage glass cartridges were utilized to effectively collect the gaseous PAHs in this study. Specifically, the cartridge was packed with 5.0 cm of XAD-16 resin sandwiched between two 2.5 cm polyurethane foam (PUF) plugs. The new cartridges were pretreated by a Soxhlet extraction process for 24 h with methanol, dichloromethane, and n-hexane for a total of 3 days. All glass fiber filters and packed cartridges were transported to and from the sampling field covered with aluminum foil to avoid PAH photo-dissociation. After sampling, all samples were extracted using a n-hexane/ dichloromethane mixed solvent of 1:1 volume ratio in Soxhlet extractors for 24 h. The total solvent volumes used were 250 and 700 mL for particulate and gaseous samples, respectively. The extract was then purged with ultra-pure nitrogen to 2 mL and passed through the cleanup column packed with silica. The eluents were then re-concentrated by purging with nitrogen to exactly 1 mL in vials. The PAH contents were determined with a gas chromatograph (GC, Agilent 6890 N) with a mass spectrometer detector (MSD, Agilent 5973 N) which was controlled by a computer workstation (Acer Aspire C500). This GC/MSD was equipped with a capillary column (HP Ultra 2; 50 m  0.32 mm  0.17 mm) and an automatic sampler (Agilent 7683 series). The masses of primary and secondary PAH ions were determined by using the scan mode for pure PAH standards. The PAHs were quantified by using the selective ion monitoring (SIM) mode. The detection limit (DL) estimated using a serial diluted standard solution of 21 PAHs was 55–735 pg. The limit of quantification (LOQ) is defined as the DL divided by the sampling volume, and was 0.109–1.47 ng m3 for individual PAH compounds in the current study. Seven consecutive injections of a 10 ng mg1 PAH standard yielded relative standard deviations (RSD) between 4.60–8.24% of the GC/MSD integration area. The R2 of calibration

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lines of the 21 PAH compounds ranged from 0.995 to 0.999. Additionally, the results showed the average recovery (n = 3) of individual PAHs by NIEA A730.70C was 80–114%. Analysis of field blanks, including glass-fiber filters and cartridges, showed all PAH levels were less than the DL. The 21 individual PAHs were divided into low-molecular weights (LM-PAHs containing two- and three-ring PAHs), midmolecular weights (MM-PAHs containing four-ring PAHs), and high-molecular weights (HM-PAHs containing five- to seven-ring PAHs) due to their different physical and chemical properties. The total-PAH data in the exhaust of a diesel-fueled engine was the sum of the 21 individual PAHs. The overall toxicity was defined as the sum of Benzo[a]pyrene equivalent (total BaPeq), which was each PAH concentration multiplied by its toxicity equivalent factor (TEF).

3. Results and discussion 3.1. Fuel stability The results of a preliminary test showed that the pure (>99.5%) n-butanol was able to mix with diesel fuel in any ratio to form a one-phase clear liquid. Unfortunately, the diesel blends became unstable with a water content of 0.5 wt.%. (Table 3), and thus, the n-butanol became essential to stabilize the water in the lipophilic diesel blends. For the 0.5 wt.% water-containing butanol (WBT) diesel blend, the results of both the 30-day and centrifugal tests showed high stability after adding 5–25 wt.% n-butanol. The above results mean that butanol with a water content of 2–9.1 wt.% (0.5/25.5–0.5/5.5) could be stably blended with diesel fuel by 5.5, 10.5, 15.5, 20.5, and 25.5% in mass. Additionally, 15–25 wt.% n-butanol could stabilize 1 wt.% water in the diesel blend, while destabilization occurred when decreasing the n-butanol content to 10 and 5 wt.% (Table 3). It is worth noting that the critical 10 wt.% water-containing butanol here becomes unstable with the addition of 11 wt.% into the diesel blend (BT10W1). Thus, a group of different additive fractions, namely 6.6, 7.7, 8.8, and 9.9 wt.%, of 10 wt.% WBT, were further tested. The results indicate that the diesel blend started to form crystalline water droplets when the additive ratio was over 6.6 wt.% WBT, and further separated into two layers after the centrifugal test. Consequently, 10 wt.% was the upper limit of water content in butanol, and lead to the formation of stable blends by adding less than 5.5 wt.% (5 wt.% pure n-butanol + 0.5 wt.%) to diesel. The fuels with a ‘‘one-phase clear’’ appearance after the centrifugal test were further examined in a heavy-duty diesel-fueled engine generator

(HDDEG). Additionally, several other fuel properties, including density, kinetic viscosity and heating value, are listed in Table 4, and will be discussed below with regard to the engine performance and emissions.

3.2. Energy performance The HDDEG was operated without any engine modifications. The suitability and efficiency could thus be determined by the brake specific fuel consumption (BSFC, mL kW1 h1), with a more efficient engine having a lower BSFC [9,16,35–37]. However, the testing device in our study was a diesel engine power generator, which had to produce electricity with a stable voltage (110 V). Therefore, the engine performance in the current research was determined by the indicated specific fuel consumption (ISFC, mL kW1 h1), which has been defined in the experimental section. The ISFC of each diesel blend increased with the increasing addition of n-butanol, as seen in Fig. 1a, and these results demonstrate an inverse correlation, with the lower heating value (LHV) decreasing as the alternative BT fractions increased. The highest butanol content (25%) thus had the highest ISFC of 533, 507, and 531 mL kW1 h1, with 0, 0.5, and 1.0 wt.% water contents, respectively. These values represent 8.0%, 2.9%, and 7.7% increases on that of D100 (493 mL kW1 h1). It is interesting that the ISFC did not keep increasing with the water content, which caused lower LHV. In theory, the ISFC increases were estimated to be 10.0%, 10.6%, and 11.3% by using BT25, BT25W0.5, and BT25W1, respectively, since the LHVs of three blends were 37.9, 37.6, and 37.4 MJ kg1. The above results indicate that the LHV was not the only dominant factor with regard to ISFC. Notably, different boiling points and volatilities of the components in blended fuel, especially those of water, would result in micro-explosions, which are reported to promote a more premixed combustion condition, enhance the energy efficiency, and reduce fuel consumption [38–44]. In the current study, the diesel blends with 0.5 wt.% water content show a significantly lower ISFC curve (Fig. 1) than those with 0 and 1.0 wt.% water. This might be because the strength of the microexplosions was enough to buffer the reduced LHV effect on ISFC at 0.5 wt.% of water, but not enough when more water (1 wt.%) was added. To more clearly analyze the combustion efficiency of each fuel blend, the indicated efficiency (IE,%), which has also been defined in Section 2, is shown in Fig. 1b. Obviously, the increasing oxygen content in BT-diesel blends leads to more complete combustion and improves the indicated efficiency of energy transformation from the fuel to electricity. Additionally, the 0.5 wt.% of water content effectively enhances the indicated efficiency by

Table 3 Blending processes and stability tests of WBTD blends. Blends

W0.5 BT25W0.5 BT20W0.5 BT15W0.5 BT10W0.5 BT5W0.5 BT25W1 BT20W1 BT15W1 BT10W1 BT5W1 BT6W0.6 BT7W0.7 BT8W0.8 BT9W0.9

n-Butanol (wt.%)

0.0 25 20 15 10 5 25 20 15 10 5 6 7 8 9

Water (wt.%)

0.5 0.5 0.5 0.5 0.5 0.5 1.0 1.0 1.0 1.0 1.0 0.6 0.7 0.8 0.9

Blending processes

Stability tests

Water/butanol (wt.%)

After stirring

After ultrasonic

After 30 days

Centrifuge

2-Clear O O O O O O Bubbles (1%) O 2-crystalline 2-crystalline 1-crystalline 1-crystalline 1-crystalline 2-crystalline

2-Emulsion O O O O O O O O 2-emulsion 2-emulsion 1-emulsion 1-emulsion 1-emulsion 2-emulsion

2-Clear O O O O O O O O 2-emulsion 2-emulsion 1-emulsion 1-emulsion 1-emulsion 2-emulsion

2-Clear O O O O O O O O 2-clear 2-clear 2-clear 2-clear 2-clear 2-clear

– 2.0 2.5 3.3 5.0 10 4.0 5.0 7.0 10 20 10 10 10 10

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3.3. NOx emissions

Table 4 Fuel properties of each diesel blend. Blends

Density

Kinetic viscosity

Heating value

D B5 B10 B15 B20 B25 B5W0.5 B10W0.5 B15W0.5 B20W0.5 B25W0.5 B15W1 B20W1 B25W1

0.830 0.829 0.828 0.827 0.826 0.825 0.830 0.829 0.828 0.827 0.826 0.829 0.828 0.827

4.152 3.191 3.241 3.292 3.342 3.393 3.180 3.231 3.281 3.332 3.382 3.270 3.321 3.372

41.66 41.23 40.80 40.37 39.94 37.86 41.02 40.59 40.16 39.73 37.65 39.96 39.53 37.44

Nitrogen oxide (NOx) emissions are the major problem with high energy efficiency and stoichiometric conditions in a direct injection diesel engine. NOx are generally formed by three pathways: (1) fuel-nitrogen conversion, (2) Zeldovich thermal activation, and (3) fuel-rich prompt formation. In comparison to regular diesel (100 ppm), all three groups of tested fuels had lower NOx levels in the exhaust, meaning that the addition of BT was effective in inhibiting the formation of NOx (Fig. 2a). By using the pure BT-diesel blends (5–25 wt.%), the NOx concentrations went up from 86 ppm (BT5) with the increasing amount of BT, and approached the highest levels (91 ppm) with the addition of 20 wt.% pure BT. The above results may be attributed to more oxidation occurring with a higher fraction of oxygenated additive, n-butanol. Furthermore, the NOx level decreased to 87 ppm when using BT25, 13.5% less than that seen with D100. This result might be due to the lower combustion temperature, which could be observed by the exhaust temperature in Fig. 2. The NOx emissions in this study were the result of a competition between the oxygen content and the combustion temperature. A similar trend was found when using 0.5 wt.% water-containing BT-diesel blends, as the absolute NOx levels of this group were all lower than those of pure BT-diesel blends. The NOx level when using BT5W0.5 was 78 ppm, representing a 22.5% reduction from that of D100. Furthermore, the NOx levels increased with the increasing addition of BT, again with the highest NOx concentration emitted when using BT15W0.5, which had 5 wt.% lower BT content than that of

Fig. 1. ISFC (a) and IE (b) of three different water content groups of tested diesel blends.

0.93–3.22% compared to the addition of pure BT. The above results indicate that butanol is an energy economic alternative to regular diesel, especially when there is a small amount of water content. However, the heating value has to be considered in practical applications for real diesel engines, as this may present a storage problem, and thus the ISFC might be the better index of the energy performance. Fortunately, the ISFCs of BT5W0.5 (495 mL kW1 h1) and BT10W0.5 (497 mL kW1 h1) did not substantially increase from D100 (493 mL kW1 h1), and thus could be better choices than the other blends.

Fig. 2. Nitrogen oxide emissions (a) and exhaust gas temperatures (b) of three different water content groups of tested diesel blends.

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the highest NOx producer with the pure BT-diesel blend (BT20). The reason why the changing blended ratio emitted the highest NOx concentration was that the existence of 0.5 wt.% water provided extra latent heat and retarded the ignition time, so that it partially approached low-temperature combustion and inhibited thermal NOx formation [45]. The exhaust temperatures when using BT20 (539 K) and BT15W0.5 (540 K) were almost the same, as shown in Fig. 2b, and this could be the critical factor with regard to thermal NOx formation. Additionally, the 1.0 wt.% water content BT-diesel blends were too unstable in 5 and 10 wt.% BT ratios to test in the engine. Thus in the group with 1.0 wt.% water, BT15W1 had the highest NOx emissions and this was reduced to the overall lowest NOx emissions of 65 ppm in the three groups of diesel blends, 35.5% lower than that of D100, by adding 25 wt.% BT to the blends. Consequently, the 1.0 wt.% water content could effectively cool the combustion system and inhibit thermal NOx formation; however, the significantly higher ISFC of BT25W1 is a major obstacle to practical use. On the other hand, BT5W0.5 had a minimal ISFC increase (+0.44%) and also a significant NOx reduction (22.5%), and thus might be a more suitable alternative diesel blend. Since the injection timing was fixed in this HDDEG, the increasing NOx levels by using low BT additive ratios could not be avoided because of their higher oxygen content. Tests of retarding injection timing are suggested in future studies, because the retarded injection will provide a more premixed air condition and absorb more heat from the compression stroke. NOx emissions are thus expected to be reduced with the lower peak temperature in the cylinder; however, the increase in ISFC will then become another problem. Low levels of BT addition are thus the better choices when retarding the injection timing, because of their lower increase in ISFC. 3.4. PM and CO emissions Particulate matter (PM) is a harmful pollutant with regard to the respiratory system and generally forms by three modes: (1) nucleation, (2) accumulation/condensation, and (3) coalescence. In the current study, the amount of n-butanol added had a positive effect in reducing PM (Fig. 3a). These trends could be due to the increasing oxygen contained in the BT additive, which further enhanced the oxidation reactions, as supported by the results of a ethanol-diesel blend investigation conducted by Lapuerta et al. [46]. Additionally, the decrease in sulfur content of the diesel blends when adding more sulfur-free additive (n-butanol) could also inhibit the formation of sulfuric acid, which has been reported to form PM by accumulating or condensing on the soot or metallic ash in exhaust gas at a lower temperature than with the in-cylinder conditions [47–49]. The PM emissions of the highest BT containing blends (BT25, BT25W0.5, and BT25W1) were 94, 52, and 88 mg Nm3, representing 31.2%, 61.9%, and 35.4% reductions from that of D100 (135 mg Nm3), respectively. According to the above sulfur-altered mechanism, BT25W1 should have the lowest PM level instead of BT25W0.5. This unexpected result might be caused by the micro-explosion mechanism, which could reduce the formation of nuclei (soot) and enhance the ISFC at the same time [42–44]. Nevertheless, the sulfur content of the tested diesel was less than 50 ppm, which would lower the effect of the sulfur-altered mechanism to reduce PM, and the regulations regarding sulfur content are now becoming stricter, e.g. a limit of 10 ppm sulfur came into effect in Europe in 2011. Thus, the mechanism of microexplosions dominates the PM reductions for different water additive ratios. Unfortunately, the PM emissions increased again by increasing the water content to 1 wt.%, which might be because the cooling effect retarded the soot oxidization and promoted the condensation pathway to form PM. Consequently, 0.5 wt.% water addition is a more efficient approach to reducing PM.

Fig. 3. Emissions of particulate matter (a) and carbon monoxide (b) of three different water content groups of tested diesel blends.

Complete combustion should transform all hydrocarbons (fuel) to water and CO2 instead of carbon monoxide (CO). However, the CO emissions increased 2.35–22.5% from D100 with the increasing amounts of BT additive, which is considered as an oxygenated fuel. This is because the oxygen content in butanol provided an oxidative condition and rapidly transformed the fuel into CO. On the other hand, the lower heating value of the BT-diesel blend led to a lower combustion temperature, slowed down the oxidation reaction from CO to CO2, and further increasing the CO level in the exhaust gas. With regard to the different water contents, the same temperature effect also occurred, with a higher water content (25 wt.%) having a higher CO emission curve than that of the lower BT-containing (0 wt.%) diesel blends. Although the CO emission increased by using BT-diesel blends, the absolute untreated levels (<313 ppm) were still much lower than the stationary source regulation (2000 ppm) in Taiwan, and were easily removed by current catalytic converters. 3.5. PAH emissions The PAHs and BaP equivalent (BaPeq) emissions of 0.5 wt.% water containing BT-diesel blends combined solid and gaseous phase PAHs, as shown in Table 5. The total-PAHs emission concentrations of 5–25 wt.% were 387, 382, 363, 353, and 319 lg Nm3, representing 6.67%, 7.92%, 12.5%, 14.9%, and 23.1% reductions, respectively, from that of D100. The major PAH compounds in the exhaust gas of the diesel engine were phenanthrene (PA) and naphthalene (Nap), which contributed 59.6–60.2% and 17.5–18.6%. These LMPAH emissions have been found to be affected by the aromatic content in a fuel [50–54]. A higher ratio of BT additive thus resulted

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Table 5 PAH emission concentration of water-containing butanol–diesel blends.

a b c d e

Conc. (lg Nm3)

D100 (n = 2)

BT5W0.5 (n = 2)

BT10W0.5 (n = 2)

BT15W0.5 (n = 2)

BT20W0.5 (n = 2)

BT25W0.5 Mean

TEFsa

Nap AcPy Acp Flu PA Ant FL Pyr CYC B(a)A CHR B(b)F B(k)F B(e)P B(a)P PER IND DBA B(b)C B(ghi)P COR P LM-PAHsb P MM-PAHsc P HM-PAHsd Total PAHs Total BaPeqe

72.5 7.81 3.73 20.7 249 9.19 12.6 31.6 0.82 1.25 1.26 1.33 1.68 0.06 0.17 0.02 0.12 0.03 0.06 0.07 0.02 363 48.9 2.24 415 1.14

69.0 7.01 3.35 19.5 232 8.19 10.9 30.0 0.78 1.15 1.18 1.24 1.60 0.06 0.15 0.01 0.11 0.02 0.06 0.05 0.02 339 45.3 2.08 387 1.05

68.2 6.81 3.00 19.3 229 8.57 10.5 29.7 0.79 1.16 1.18 1.25 1.61 0.05 0.15 0.02 0.09 0.02 0.05 0.05 0.02 335 44.6 2.07 382 1.05

66.8 6.73 2.72 19.1 216 5.76 10.0 29.0 0.81 1.18 1.22 1.11 1.66 0.05 0.16 0.02 0.04 0.02 0.05 0.04 0.02 317 43.4 2.08 363 1.00

65.8 6.13 2.36 18.9 211 5.19 10.1 27.5 0.76 1.02 1.11 1.05 1.56 0.05 0.15 0.01 0.03 0.02 0.04 0.04 0.02 309 41.6 1.92 353 0.94

59.1 5.82 2.16 17.2 190 4.74 8.37 25.3 0.79 1.07 1.17 1.06 1.62 0.04 0.14 0.01 0.03 0.02 0.04 0.04 0.02 279 37.8 1.96 319 0.91

0.001 0.001 0.001 0.001 0.001 0.010 0.001 0.001 0 0.100 0.010 0.100 0.100 0 1.000 0 0.100 1.000 0 0.010 0 – – – – –

TEF is abbreviated from ‘‘Toxicity Equivalency Factor’’. LM is abbreviated from ‘‘Low-Molecular’’. MM is abbreviated from ‘‘Medium-Molecular’’. HM is abbreviated from ‘‘High-Molecular’’. BaPeq means the BaP toxicity equivalency of PAHs.

in lower LM-PAH emissions, and further significantly reduced the total-PAH levels. However, the toxicities of LM-PAHs (TEF = 0.001) are much lower than those of MM- and HM-PAHs (TEF = 0.01–1.0). Therefore, reducing the MM- and HM-PAHs is more efficient with regard to reducing the total-BaPeq. In the current investigation, the reductions in MM- and HM-PAHs were 7.31–22.7% and 7.02–1.44%, respectively, with different water contents. These can be attributed to the lower PM concentration, as it has been reported to be a MM- and HM-PAH adsorbent. In addition, the reductions of all molecular weight PAHs emissions could be partially because the higher oxygen content in BT-diesel blends promoted more complete combustion, retarded the LM-PAH formation, especially the precursor (benzene), and accelerated the MM- and HM-PAH degradation [55]. Nevertheless, the micro-explosions that could improve the atomization of fuel droplets also accelerated the oxidation of PAHs [7,42,44]. Therefore, the total BaPeq emitted was 0.91–1.14 lg Nm3, representing 7.78–20.4% reductions from that of D100 by using BT-diesel blends with 0.5 wt.% water content. 4. Conclusion (1) The diesel blends with 0 and 0.5 wt.% water content could remain as a one-phase clear liquid with 5–25 wt.% n-butanol added, while those blends with 1.0 wt.% water needed at least 15 wt.% n-butanol added as a stabilizer. (2) The ISFC and CO emissions increased with the increasing BT fractions, while the PM, total-PAHs, and total-BaPeq levels reduced in HDDEG. The oxygen contained in BT provided an oxidative condition to rapidly form CO and inhibit the formation of PM. The lower combustion temperature achieved by adding BT caused the reduction in NOx, but also increased the CO. The above trends, which were caused by lower combustion temperature, are all supported by the findings of Park et al. [56], who focused on bioethanol–blended diesel.

(3) NOx emissions were inversely related to the exhaust temperatures with higher BT additions (15–25 wt.%), while BT20, BT15W0.5, and BT15W1 had the maximum NOx emissions in the 0, 0.5, and 1.0 wt.%-containing blends, respectively. The major mechanism of NOx reduction was the lower combustion temperature [56] achieved by adding BT and water. (4) The 0.5 wt.% water content showed a dramatic reduction of ISFC from pure and 1.0 wt.% BT addition, which is explained by the fact that the micro-explosions could provide better atomization and moderate the ISFC of low-heating value BT-diesel blends. (5) The ISFC of BT5W0.5 (495 mL kW1 h1) and BT10W0.5 (497 mL kW1 h1) did not substantially increase from that of D100 (493 mL kW1 h1). In addition, they reduced NOx by 22.5% and 16.5%, PM by 41.1% and 44.9%, total-PAHs by 6.67% and 7.92%, and total-BaPeq by 7.91% and 7.78%, respectively, compared to those of regular diesel. In order to consider both energy performance and pollutant emissions, the 5 and 10 wt.% BT additives with 0.5% water content were the two most suitable blends for practical use in an HDDEG. Nevertheless, butanol additive can be utilized by applying bio-processes that can reduce CO2 emissions over a product’s life cycle, and which are more environmentally friendly than those associated with petro-diesel.

Acknowledgments This research was supported in part by the National Science Council in Taiwan, Grant Number NSC 98-2621-M-006-007. The authors also thank Professor S.J. Chen, Dr. S.W. Li, Mr. J.H. Tsai, Mr. Y.C. Liu, Ms. L.P. Chang, and Mr. S.S. Chang for their insightful discussions and great help in the experimental work.

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