- Email: [email protected]
Particulate emission characterization of a biodiesel vs diesel-fuelled compression ignition transport engine: A comparative study
ARTICLE IN PRESS
Atmospheric Environment 40 (2006) 5586–5595 www.elsevier.com/locate/atmosenv
Particulate emission characterization of a biodiesel vs diesel-fuelled compression ignition transport engine: A comparative study Dipankar Dwivedia, Avinash Kumar Agarwalb,, Mukesh Sharmaa a
Department of Civil Engineering, IIT Kanpur, Kanpur 208016, India Department of Mechanical Engineering, IIT Kanpur, Kanpur 208016, India
b
Received 9 March 2006; received in revised form 1 May 2006; accepted 2 May 2006
Abstract This study was set out to characterize particulate emissions from diesel engines fuelled by (i) mineral diesel and (ii) B20 (a blend of 20% biodiesel with diesel); in terms of metals and benzene soluble organic fraction (BSOF), which is an indicator of toxicity and carcinogenicity. A medium duty, transport diesel engine (Mahindra MDI 3000) was operated at idling, 25%, 50%, 75% and rated load at maximum torque speed (1800 rpm) and samples of particulate were collected using a partial flow dilution tunnel for both fuels. Collected particulate samples were analyzed for their metal contents. In addition, metal contents in mineral diesel, biodiesel and lubricating oil were also measured to examine and correlate their (metals present in fuel) impact on particulate characteristics. Results indicated comparatively lower emission of particulate from B20-fuelled engine than diesel engine exhaust. Metals like Cd, Pb, Na, and Ni in particulate of B20 exhaust were lower than those in the exhaust of mineral diesel. However, emissions of Fe, Cr, Ni Zn, and Mg were higher in B20 exhaust. This reduction in particulate and metals in B20 exhaust was attributed to near absence of aromatic compounds, sulphur and relatively low levels of metals in biodiesel. However, benzene soluble organic fraction (BSOF) was found higher in B20 exhaust particulate compared to diesel exhaust particulate. r 2006 Elsevier Ltd. All rights reserved. Keywords: Particulate; Biodiesel; Metals; Benzene soluble organic fraction; Toxicology
1. Introduction Diesel engines are a major source of nitrogen oxides and particulate, which mainly consist of soot and metals. The composition varies depending on engine type, operating conditions, fuel and lubricatCorresponding author. Tel.: +91 512 2597982; fax: +91 512 2597982. E-mail address: [email protected] (A.K. Agarwal).
1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.05.005
ing oil composition and whether an emission control system is present. Recent studies have focused on the composition and toxicity of diesel exhaust (DE) and diesel particulate matter (DPM) (USEPA, 2002; Sharma et al., 2005). The USEPA and other agencies, engine and vehicle manufacturers, emission control system manufacturers, and fuel refiners have been working for the past few decades to substantially reduce emissions from diesel engines. The chemistry and properties of
ARTICLE IN PRESS D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
diesel fuel have a direct effect on emissions and formation of pollutants from diesel engines (EPA, 2001). Scientists are constantly working on alternative fuels, which are clean and efficient in combustion. These fuels include compressed natural gas (CNG), biodiesel, alcohols, gas-to-liquid fuels (GTL), dimethyl-ether (DME), etc. For example, CNG has been extensively used as a clean fuel in Delhi and several other cities of the world. There is a need to study possible usage of other alternative fuels at large scale and their impact on human health and environment. It is not feasible to replace diesel engines with CNG engines in cities all over the world. Other alternative fuels need to be examined for engine performance and emission characteristics. Biodiesel is one such fuel which is a carbonneutral fuel from bio-origin. B20 is a blend of 20% biodiesel and 80% mineral diesel, which may partially replace mineral diesel and can be implemented without significant changes in the existing engine hardware. Biodiesel has also shown potential to reduce the problem of CO2 emissions and can partly meet energy needs in rural areas (Wedel, 1999). It needs to be recognized that biodiesel does provide an effective alternative to diesel, but unless it is ensured that emissions from biodiesel (especially toxic pollutants) will be lower, or same as diesel, acceptability of biodiesel as a fuel on a large scale may not be forthcoming. Biodiesel for the present experimental study has been made from rice-bran oil using the process of transesterification using methanol and basic catalyst (KOH). The physical properties of biodiesel (ricebran oil methyl ester), mineral diesel and B20 are shown in Table 1. Out of metals studied in this research (Cr, Ni, Pb, Cd, Na, Al, Mg and Fe) Cr, Ni, Pb, and Cd are toxic metals having a limit of maximum allowable exposure. The acceptable ambient air concentration for Ni is 5 ng m 3, Pb is 1000 ng m 3 and for Cd, it
5587
is 20 ng m 3. Fe, Al, and Mg are generally present in soil in the form of crust metals but excess of these metals in atmosphere is of concern. In addition, profiling of metal content in engine exhaust is necessary to identify and apportion air-pollution sources. The focus of this paper is on comparative assessment and characterization of emissions from traditional diesel and B20 (20% biodiesel blend) fuel exhausts in terms of (i) particulate, (ii) metals in particulate, and (iii) benzene soluble organic fraction (BSOF; a toxicity indicator) in particulate. 2. Materials and methods To characterize the emissions from mineral diesel and B20, a typical medium duty transport engine (Model: MDI 3000 A; Make: Mahindra and Mahindra Ltd., India) was used in present experimental investigations (Fig. 1). This is a four-cylinder, four-stroke, variable-speed, transport engine with direct-injection of fuel. Detailed specifications of the engine are given in Table 2. This engine is installed with an eddy-current dynamometer (Model: ASE-70; Make: Shenck-Avery India Ltd.). The eddy-current dynamometer is equipped with a dynamometer controller capable of loading the engine at the desired speed/load. For collection of particulate for characterization, the engine was operated at loads ranging from idling, 25%, 50%, 75%, to rated engine load at a constant speed of 1800 rpm (rated speed for maximum torque). Particulate samples were collected iso-kinetically using a partial flow dilution tunnel, which was designed and fabricated Fuel Tank
Eddy Current Dynamometer Controller
Air Box
Engine
Table 1 Physical properties of diesel and biodiesel Fuel property
Diesel
B100
B20
Specific gravity at 30 1C Viscosity (cSt) at 40 1C Calorific value (MJ kg 1) Flash point (1C) Sulfur content (ppm)
0.839 3.18 44.8 48 500
0.877 5.30 42.2 265 10
0.847 3.48 44.1 — 400
Exhaust Calorimeter Fig. 1. Mahindra (MDI-3000 A) engine and dynamometer system.
ARTICLE IN PRESS D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
5588 Table 2 Engine specifications Manufacturer
Mahindra & Mahindra Ltd., India
Model No. of cylinders, configuration Combustion system Bore/stroke (mm) Engine displacement (cc) Compression ratio Maximum speed at full load (RPM) Rated power (RPM)
MDI 3000 A Four, in-line Direct injection 88.9/101.6 2520 18:1 2300 40 hp (2300)
layer, which changes the structure, composition and density of the particulate. Partial flow dilution tunnel is used to simulate this near field (o3 m) mixing, condensation and adsorption process in the present study. When the diesel engine is operated, carbonaceous soot particles and high boiling point hydrocarbons are emitted from the tailpipe. These hydrocarbons condense on soot to form particulate matter (PM) after being diluted with preheated air inside the partial flow dilution tunnel. The dilution ratio is typically kept at 10:1. 2.3. Test procedure
Table 3 Sampling details Samples collected for analysis
Load
Sampling duration (min.)
Benzene soluble organic fraction (single sample)
Idle
30
25% 50% 75% 100%
25 25 25 25
Idle 25% 50% 75% 100%
30 25 25 25 25
Metals (duplicate sample)
in-house for this purpose (Dwivedi, 2005), described later. The gaseous emissions were measured using a raw exhaust gas emission analyzer (Model: EXSA1500; Make: HORIBA Ltd., Japan) however this paper discusses the results of particulate and BSOF only. 2.1. Diesel engine and dynamometer system Particulate sample were collected on filter paper (GF/A, 47 mm. Make: Nupore Filtration System, Batch No. 1720704) for analyzing various metals and BSOF. Sampling details are given in Table 3. 2.2. Standard sampling method: dilution tunnel The particulate leaving the exhaust pipe are at relatively high temperature. These particulate and gases which contain hydrocarbons also cool down during the mixing and dilution process with the atmospheric air, and the associated condensation of hydrocarbons takes place on the particulate surface
The first step in the sampling procedure is preparation of filter papers. The filter papers were desiccated for 12 h and then weighed. One filter paper was placed in the filter holder and then the filter assembly was installed in the partial flow dilution tunnel. After running the engine at desired load and speed condition and collecting the particulate through partial flow dilution tunnel for a predetermined period of time, the filter papers were removed from the filter assembly and again kept in desiccators for 12 h and then weighed. Particulate emission was found gravimetrically by measuring difference in weights of filter paper, before and after the particulate sampling. These filter papers were then analyzed for characterization of particulate for metals and BSOF. 2.4. Instruments and measurement systems 2.4.1. Extraction of samples for metal analysis in particulate Extraction of samples for metals analysis was carried out using the hot plate method. In hot acid extraction (USEPA, 1995), the filter strips are placed in a beaker and extracted by refluxing on a hot plate, using 10 mL hydrochloric acid (8%) nitric acid (3%) solution. The digested solution was filtered before analysis. 2.4.2. Sample preparation for analyzing metals in diesel and lubricating oil Sample preparation for metals analysis was carried out using acid digestion of oils for metals analysis by atomic absorption spectrometry (AAS). A representative 0.5 g sample was mixed with 0.5 g of finely ground potassium permanganate and then 1.0 mL of concentrated sulfuric acid was added while stirring. A strong exothermic reaction takes
ARTICLE IN PRESS place. The sample was then treated with 2 mL concentrated nitric acid. Ten mL of concentrated HCl was added and the sample was heated until the reaction was complete and then the metal extract was filtered for further analysis using AAS (USEPA, 2002). 2.4.3. Estimation of BSOF in particulate Analysis for BSOF of the particulate samples was carried out using ASTM test method D 4600-87 (ASTM, 2001). This method is also recommended by the National Institute of Occupational Safety and Health, USA to represent the toxic organic compounds in the particulate. This test method describes the sampling and gravimetric determination of benzene-soluble PM.
Particulates ( mg/m3)
D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
5589
55 45 35 Diesel
25
Biodiesel
15 0%
25%
50%
75%
100%
Engine Load (%) Fig. 2. Variation in particulate emission with engine load.
Table 4 Concentration of various metals in diesel, biodiesel and lubricating oil samples
2.5. Metal analysis using AAS Diesel
AAS (Model: GBC Avanta S, Australia) was used to carry out analysis of samples of diesel, biodiesel and lubricating oil along with particulate samples collected on filter papers. Metals analyzed included Fe, Mg, Cr, Ni, Pb, Zn, Cd, Al, and Na. For analyzing these metals, first standard solutions of known concentration of salts containing these metals were prepared and instrument was calibrated.
Biodiesel
Lubricating oil
247.5 180.5 10.2 15.8 ND ND ND 2.2 1.85
1770 656.55 20.1 27.3 ND ND ND 35.1 1.45
(mg g 1) Na Zn Fe Mg Ni Pb Cd Al Cr
1393.75 152.1 7 9.2 ND 0.93 0.52 1.55 1.3
3. Results and discussion Particulate samples were collected from the compression ignition engine using diesel and B20 and were analyzed for metals and BSOF as per the sampling plan shown in Table 3. This section presents a summary and interpretations of the experimental results for DE and 20% biodiesel exhaust (BDE). 3.1. Particulate One of the objectives of this study was to examine particulate emission in DE and BDE under varying engine load conditions. Fig. 2 shows variations of particulate emission under varying engine loads for diesel and B20 fuel at constant engine speed. Shobokshy (1984) and Sharma et al. (2005) have reported that particulate concentration increases with increased engine load; the same trend is obtained in present study for both the fuels (Fig. 2). However, it is noteworthy that particulate emission is higher in DE
(22–59 mg m 3) than B20 exhaust (17–48 mg m 3). Studies by Sharp (1996) and Wedel (1999) established that a B20 blend (approximately 2% oxygen for RME-20) reduces particulate by approximately 30% compared to particulate in the DE. Fig. 2 shows that rate of increase (with increasing load) of particulate emission is lower for B20 exhaust. This lower increase in particulate concentration in B20 exhaust can be attributed to (i) higher oxygen content in B20 and (ii) lower C/H ratio; in B20 C/H ratio 6.53 compared to diesel 6.82 (Turrio-Baldassarri et al., 2004; Canakci and Van Gerpen, 2001), and (iii) near absence of sulphur and aromatic content of biodiesel. 3.2. Metals in mineral diesel, biodiesel and lubricating oil Table 4 shows measured levels of Fe, Cr, Ni, Pb, Zn, Cd, Na, and Al in diesel, biodiesel (before blending with mineral diesel) and lubricating oil.
ARTICLE IN PRESS D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
5590
The basic metal content in the fuels and lubricating oil are variable e.g. iron is found to be higher in biodiesel, whereas lead and cadmium is not detected. The variable metal contents will have an impact on the metal in the particulate exhaust of diesel and biodiesel. There are other factors like lubricity of the fuel, which affects the wear of fuel injection system and presence of wear metals in particulate to a minor degree.
3.3. Metals in particulate collected from DE and BDE The experimental study showed that concentrations of Fe, Mg, and Na (crust elements) were much higher than those of Cr, Ni, Pb, Zn, Al and Cd (anthropogenic elements) in both the fuels. The similar trend has also been reported by Wang et al. (2000). The results of present experimental investigation for metals in particulate are divided in two sections based on (i) relatively higher concentration in DE and (ii) relatively higher concentration in BDE.
3.4. Metals present in higher concentration in DE Four metals namely lead, cadmium, sodium and nickel are present in higher concentration in the particulate collected from DE compared to BDE (Fig. 3). Metal-wise emissions are discussed below. 3.4.1. Lead It may be noted from Fig. 3 that there are lower emissions of lead in BDE than in DE (0.9–0.30 mg g 1 for DE, 0.7–0.25 mg g 1 for BDE). The analysis of lead concentration in diesel, biodiesel and lubricating oil depicted that it was present only in diesel and was not present in biodiesel and lubricating oil (Table 4). With B20 blend, the emission of lead decreased because absence of lead in biodiesel. 3.4.2. Cadmium There is a slight reduction in emissions of cadmium for BDE compared to DE (0.13–0.027 mg g 1 for DE, 0.12–0.024 mg g 1 for BDE). The analysis of cadmium concentration in diesel, biodiesel and lubricating oil showed that it was present in diesel but was non-detectable in
0.9
Diesel
0.8
B20
Diesel
0.13 Cadmium (mg/g)
Lead (mg/g)
1
0.7 0.6 0.5 0.4
B20 0.09
0.05
0.3 0.2
0.01 0%
25%
50% 75% Engine Load (%)
100%
0%
25%
50% 75% Engine Load (%)
100%
1.4 1.2
Diesel
10
B20
Nickel (mg/g)
Sodium (mg/g)
12
8 6 4 2
1 0.8 0.6 Diesel
0.4
B20
0.2
0 0%
25%
50% 75% Engine Load (%)
100%
0 0%
25%
50% 75% Engine Load (%)
Fig. 3. Concentration of various metals present in higher concentration in particulate from diesel exhaust.
100%
ARTICLE IN PRESS D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
biodiesel and lubricating oil (Table 4). The source of cadmium in DE and BDE is possibly from fuel (mineral diesel) and engine wear. 3.4.3. Sodium It may be noted that sodium shows highest emissions (12–2 mg g 1 for DE, 9–1 mg g 1 for BDE) amongst all crust metals. This is due to the fact that sodium is present in abundance in diesel, biodiesel and lubricating oil (Table 4). Presence of sodium was found highest in lubricating oil followed by diesel and biodiesel. With B20 fuel, the emission of sodium decreased because biodiesel contains lower amount of sodium as compared to mineral diesel. It is seen in Fig. 3 that there is significant difference in sodium content in particulate from DE and BDE at idling. In this study, a small bore (88.9 mm) engine is used. In smaller engines, there is always a possibility of fuel dilution of lubricating oil because of fuel spray impingement on lubricated cylinder walls. This is an undesirable situation however this invariably happens in smaller diesel engines. Fuel lubricity plays an important role in injection system wear and additional lubricity properties of fuel (biodiesel) leads to lower injection system wear (Bijwe et al., 2004). 3.4.4. Nickel Nickel shows emissions in DE and was not detected in case of BDE (1.3–0.7 mg g 1 for DE, Not detected for BDE). As shown in Fig. 3, nickel was either absent or was below detection level in both the fuels and lubricating oil. It can therefore be inferred that whatever nickel is coming out in DE may be from engine wear. With B20, the emission of nickel in particulate is not observed possibly due to self-lubricity property of biodiesel, which results in reduced engine and fuel injection system wear. 3.5. Metals present in higher concentration in BDE Other metals such as iron, aluminum, zinc, chromium and magnesium are found in higher concentration in the particulate collected from BDE compared to DE (Fig. 4). 3.5.1. Iron Fig. 4 presents iron content in particulate drawn from DE and BDE at different engine loads. It may be noted that iron shows highest emission (7–2 mg g 1 for DE, 8–3 mg g 1 for BDE) amongst
5591
all anthropogenic metals. This is due to the fact that iron is present in abundance in diesel, biodiesel and lubricating oil. In addition, iron may also be contributed from engine wear (Agarwal et al., 2003). With B20, the emission of iron is found to be higher because biodiesel contains higher amount of iron compared to mineral diesel (Table 4). 3.5.2. Aluminum It may be noted that aluminum shows higher emissions for biodiesel operation (2.2–0.9 mg g 1 for DE, 2.3–1.5 mg g 1 for BDE; Fig. 4). Aluminum content is higher in biodiesel compared to mineral diesel (Table 4); hence it is reflected in higher content in particulate from BDE. 3.5.3. Zinc It may be noted that zinc concentrations vary from 7.8 to 2.3 mg g 1 for DE and 8.2–3.1 mg g 1 for BDE (Fig. 4). With B20, the emission of zinc increased because biodiesel contains more zinc compared to mineral diesel. There is no significant difference in zinc concentration in particulate from DE and BDE at idling. As load increases the difference seems to be more pronounced because at idling, consumption of lubricating oil is lower for biodiesel due to self-lubricity property of biodiesel. 3.5.4. Chromium It may be noted that chromium shows higher emissions for BDE compared to DE (1.2–0.4 mg g 1 for DE, 1.5–0.6 mg g 1 for BDE; Fig. 4). The analysis of chromium concentration in diesel, biodiesel and lubricating oil shows that its presence in biodiesel is higher than mineral diesel (Table 4). With B20, the emission of chromium increased as compared to diesel because of its higher concentration in fuel itself (Table 4). Difference between the concentration of lubricating oil and biodiesel is not very high so lubricating oil does not play any significant role in emission of chromium. 3.5.5. Magnesium Magnesium concentration varies from 8.7 to 5 mg g 1 for DE and 9.12–6.2 mg g 1 for BDE (Fig. 4). This is due to the fact that magnesium is present in abundance in diesel, biodiesel and lubricating oil (Table 4). It is clear from Fig. 4 that there is not much difference in magnesium content for DE and BDE at idling. As load increases this difference gets pronounced. As load further in-
ARTICLE IN PRESS D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
5592
2.8
8 Iron (mg/g)
Aluminium (mg/g)
Diesel
7
B20
6 5 4 3 2 1
Diesel B20
2.4 2 1.6 1.2 0.8
0%
25%
50%
75%
100%
0%
25%
Engine Load (%)
75%
100%
1.6
8 Diesel B20
7 6
Chromium (mg/g)
ZInc (mg/g)
50% Engine Load (%)
5 4 3
1.4
Diesel B20
1.2 1 0.8
e
0.6 0.4
2
0.2 0%
25%
75%
50%
0%
100%
25%
Magnesium (mg/g)
50%
75%
100%
Engine load (%)
Engine Load(%)
9.5 9 8.5 8 7.5 7 6.5 6 5.5 5
Diesel B20
0%
25%
50%
75%
100%
Engine Load (%) Fig. 4. Concentration of various metals present in higher concentration in particulate from B20 exhaust.
creases, the consumption of lubricating oil also increases. Lubricating oil has a large content of magnesium in organo-metallic additives, which are responsible for the increase in magnesium content in DE and BDE. 3.6. Source of metals in DE and BDE: overall characterization of metal emissions Fig. 5 presents metal levels in particulate from DE and BDE and metals present in fuels. Fe, Mg, Cr, Pb, Al and Cd show a good association between metals in
fuels and metals in exhaust particulate. In B20 fuel, concentration of metals is calculated by mass balance (80% metal content in diesel added with 20% metal content of biodiesel). Zn and Na do not show any such association, it can be inferred from this investigation that sources of Zn and Na in exhaust particulate may be from sources other than fuel. The metal concentration results presented in Figs. 3 and 4 reflect that as load increases, the metal content in particulate gradually decreases. This can be explained by the fact that at higher engine load, combustion takes place at higher
ARTICLE IN PRESS
5 4 3 2 1
12
Mg
Cr
Ni
Pb
Al
7 B20 Biodiesel Exhaust
10
6 5
8
4
6
3 4
2
2
1
0
0 Fe
5593
0 Fe
Cd
Mg
Metals
Cr
Ni
Pb
Al
Metal Content in Diesel (ug/g)
6 Diesel Diesel Exhaust
Metal Content in Particulates (mg/g)
10 9 8 7 6 5 4 3 2 1 0
Metal Content in Diesel (ug/g)
Metal Content in Particulates (mg/g)
D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
Cd
Metals Fig. 5. Sources of metals in diesel and biodiesel exhaust.
60 Diesel Biodiesel
50 BSOF (% w/w)
temperature; leading to improved thermal efficiency. In other words, it is known that the emission of particulate matter is strongly affected by the operating conditions of the engine. In particular, lower engine load/ speed might result in lower thermal efficiency and hence lead to higher particle formation and emission. Brake specific fuel consumption of diesel engine decreases with increasing load/ speed and this is reflected in reduced emission of metals with increasing engine load. It is also reported by Sharma et al. (2005) that overall particulate formation increases in the form of elemental carbon with increase in engine load. Particulate matter emission is higher for higher engine loads because higher amount of fuel is being injected and burnt in the engine. For this reason, the particulate emission from the diesel engine is higher but the corresponding metal content is lower for higher engine load as evident for almost all metals. It must be recognized that at higher engine loads, particulate emission is more in terms of elemental carbon; thereby reducing metal content (mg g 1) in particulate (Ullman, 2004). This is noteworthy that the thermal efficiency of the engine increases with increasing engine load and after a threshold limit (in present study it corresponds to approximately 75–85% of rated load), efficiency starts decreasing. Emission of metals increases at 100% load (compared to 75% load) for almost all metals, due to higher specific brake fuel consumption at full load (Figs. 3 and 4). In view of the above discussions, this can be concluded that metal content in diesel, biodiesel and lubricating oil play an important role in the emission of metals in the engine exhaust and
40 30 20 10 0 0%
25%
50%
75%
100%
Load (%) Fig. 6. Variation of BSOF with engine load.
blending with biodiesel is likely to result in lower emissions of metals. 3.7. BSOF in particulate BSOF has been taken as indicator of organic fraction of particulate which represents the toxic compounds. It can be observed from Fig. 6 that at idling, BSOF was about 48% in DE and 55% in BDE and BSOF gets lowered with increase in engine load. At idling condition, the mineral diesel and lubricating oil undergo partial combustion (pyrolysis) due to low temperature conditions prevailing in the combustion chamber. This leads to higher unburnt hydrocarbon species formation and emissions, which is detected as BSOF (Sharma et al., 2005). Although the exact mechanism for presence of BSOF varies depending on the engine’s operating
ARTICLE IN PRESS 5594
D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
condition, generally accepted explanation for BSOF in exhaust is that soot particles that are formed in fuel-rich portions of the diesel (spray) collect hydrocarbons through condensation and adsorption (Finch et al., 2002). Higher molecular weight compounds will form small liquid droplets and get adsorbed and deposited on the carbonaceous soot particles. These small droplets may be adsorbed by soot particles or simply be trapped on the filters used to collect particulate. Operating conditions which increase the fraction of the fuel and lubricating oil that remains unburned or partially burned; will increase the BSOF of the particulate. For B20 particulate, this value of BSOF is higher than that of diesel particulate. This is primarily because of lower volatility of constituents of biodiesel; biodiesel can be expected to increase the amount of soluble organic fraction emitted by a compression ignition engine. However, the increased mass consists mostly of unburned esters from the fuel itself (Sharp et al., 2000). Since biodiesel is nontoxic, the increased level of soluble organic fraction may not be hazardous (Finch et al., 2002). However in order to clearly establish the organic toxicity of BDE, more investigations are required through further speciation of exhaust. 4. Conclusions Oxygenated fuel B20 (biodiesel blend) showed superior engine performance in reducing particulate emissions at all operating conditions compared to mineral diesel (particulate in DE; 22–59 mg m 3 and in BDE; 17–48 mg m 3) at constant speed. This may be due to lower sulphur and aromatic content of biodiesel. Along with reduction in particulate matter, there is an overall reduction in metal emissions from the biodiesel exhaust. It was also found that metals in particulate mainly originate from fuel and lubricating oil and in addition some of the metals can originate from engine wear. The reduced metal emissions in BDE are because of (i) inherent lubricity property of biodiesel leading to reduction of emissions of certain metals in BDE due to lower fuel injection system wear and (ii) lower metal levels (Pb and Cd) in biodiesel compared to mineral diesel. There is a net increase in BSOF for B20 fuel as compared to mineral diesel, however biodiesel is nontoxic, hence the increased level of soluble organic fraction may not be viewed as hazardous (Finch et al., 2002). However hazardous nature of
biodiesel exhaust needs further investigations by speciating organic species in biodiesel exhaust. This necessitates that speciation of organic compounds is done for both the fuel exhausts to clearly establish comparative organic toxicity. Nonetheless, this research suggests that overall toxicity of emissions in terms of metals reduces in BDE compared to DE.
References Agarwal, A.K., Bijwe, J., Das, L.M., 2003. Effect of biodiesel utilisation on wear of vital parts in compression ignition engine. Journal of Engineering for Gas Turbine and Power 125, 604–611. ASTM, 2001. D-4600-87: Standard test method for determination of benzene soluble particulate matter in workplace atmosphere. Bijwe, J., Agarwal, A.K., Sharma, A., 2004. Assessment of lubricity of biodiesel blends in reciprocating wear mode. Journal of Fuels & Lubricants, Section 4, 2117–2126 SAE Paper No 2004-01-3068, SAE Transactions 2004. Canakci, M., Van Gerpen, J.H., 2001. Comparison of engine performance and ignition for petroleum diesel fuel, yellow grease biodiesel, and soy bean oil biodiesel. ASAE 0160050, Annual International Meeting, CA, USA. Dwivedi D., 2005. A comparative study of particulate & emission characterization from diesel and biodiesel exhausts. M.Tech. Thesis, submitted to Department of Civil Engineering, IIT Kanpur, India. EPA 420-P-01-001, 2001. Strategies and Issues in Correlating Diesel Fuel Properties with Emissions, July. Finch, G.L., et al., 2002. Effects of subchronic inhalation exposure of rats to emissions from diesel engine burning soybean oil-derived biodiesel fuel. Inhalation Toxicology 14, 1017–1048. Sharma, M., Agarwal, A.K., Bharathi, K.V.L., 2005. Characterization of exhaust particulate from diesel engine. Atmospheric Environment 39, 3023–3028. Sharp, C.A., 1996. Emissions and Lubricity Evaluation of Rapeseed Derived Biodiesel Fuels. Southwest Research Institute, San Antonio, TX. Sharp, C.A., Howell, S.A., Jobe, J., 2000. The effect of biodiesel fuels on transient emissions from modern diesel engines, Part II—unregulated emissions and chemical characterization. SAE Paper No. 2000-01-1968. Shobokshy, M.S.E., 1984. A preliminary analysis of the inhalable particulate lead in the ambient atmosphere of the city of Riyadh, Saudi Arabia. Atmospheric Environment 18 (10), 2125–2130. Turrio-Baldassarri, L., Battistelli Chiara, L., Conti, L., Crebelli, R., Berardis Barbara, D., Iamiceli, A.L., Gambino, M., Iannaccone, S., 2004. Emission comparison of urban bus engine fueled with diesel oil and biodiesel blend. Science of the Total Environment 327, 147–162. Ullman, T.L., 2004. Diesel Emission Testing. Asian Vehicle Emission Control Conference, Beijing, China. USEPA, 1995. Health assessment document for diesel engine exhausts. EPA/600/8-90/057, US Environmental Protection Agency.
ARTICLE IN PRESS D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595 USEPA, 2002. Health assessment document for diesel engine exhaust. Washington, DC. Wang, W.G., Lyons, D.W., Clark, N.N., Gautam, M., Norton, P.M., 2000. Emissions from heavy trucks fueled by diesel and biodiesel blend without engine modifications. Environmental Science and Technology 34, 933–939.
5595
Wedel, R.von., 1999. Marine Biodiesel in Recreational Boats, second ed. CytoCulture International, Inc., Point Richmond, CA and 22 April, 1999. (Marine Biodiesel and Education Project for San Francisco Bay and Northern California. Prepared for the National Renewable Energy Laboratory US Department of Energy).
Atmospheric Environment 40 (2006) 5586–5595 www.elsevier.com/locate/atmosenv
Particulate emission characterization of a biodiesel vs diesel-fuelled compression ignition transport engine: A comparative study Dipankar Dwivedia, Avinash Kumar Agarwalb,, Mukesh Sharmaa a
Department of Civil Engineering, IIT Kanpur, Kanpur 208016, India Department of Mechanical Engineering, IIT Kanpur, Kanpur 208016, India
b
Received 9 March 2006; received in revised form 1 May 2006; accepted 2 May 2006
Abstract This study was set out to characterize particulate emissions from diesel engines fuelled by (i) mineral diesel and (ii) B20 (a blend of 20% biodiesel with diesel); in terms of metals and benzene soluble organic fraction (BSOF), which is an indicator of toxicity and carcinogenicity. A medium duty, transport diesel engine (Mahindra MDI 3000) was operated at idling, 25%, 50%, 75% and rated load at maximum torque speed (1800 rpm) and samples of particulate were collected using a partial flow dilution tunnel for both fuels. Collected particulate samples were analyzed for their metal contents. In addition, metal contents in mineral diesel, biodiesel and lubricating oil were also measured to examine and correlate their (metals present in fuel) impact on particulate characteristics. Results indicated comparatively lower emission of particulate from B20-fuelled engine than diesel engine exhaust. Metals like Cd, Pb, Na, and Ni in particulate of B20 exhaust were lower than those in the exhaust of mineral diesel. However, emissions of Fe, Cr, Ni Zn, and Mg were higher in B20 exhaust. This reduction in particulate and metals in B20 exhaust was attributed to near absence of aromatic compounds, sulphur and relatively low levels of metals in biodiesel. However, benzene soluble organic fraction (BSOF) was found higher in B20 exhaust particulate compared to diesel exhaust particulate. r 2006 Elsevier Ltd. All rights reserved. Keywords: Particulate; Biodiesel; Metals; Benzene soluble organic fraction; Toxicology
1. Introduction Diesel engines are a major source of nitrogen oxides and particulate, which mainly consist of soot and metals. The composition varies depending on engine type, operating conditions, fuel and lubricatCorresponding author. Tel.: +91 512 2597982; fax: +91 512 2597982. E-mail address: [email protected] (A.K. Agarwal).
1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.05.005
ing oil composition and whether an emission control system is present. Recent studies have focused on the composition and toxicity of diesel exhaust (DE) and diesel particulate matter (DPM) (USEPA, 2002; Sharma et al., 2005). The USEPA and other agencies, engine and vehicle manufacturers, emission control system manufacturers, and fuel refiners have been working for the past few decades to substantially reduce emissions from diesel engines. The chemistry and properties of
ARTICLE IN PRESS D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
diesel fuel have a direct effect on emissions and formation of pollutants from diesel engines (EPA, 2001). Scientists are constantly working on alternative fuels, which are clean and efficient in combustion. These fuels include compressed natural gas (CNG), biodiesel, alcohols, gas-to-liquid fuels (GTL), dimethyl-ether (DME), etc. For example, CNG has been extensively used as a clean fuel in Delhi and several other cities of the world. There is a need to study possible usage of other alternative fuels at large scale and their impact on human health and environment. It is not feasible to replace diesel engines with CNG engines in cities all over the world. Other alternative fuels need to be examined for engine performance and emission characteristics. Biodiesel is one such fuel which is a carbonneutral fuel from bio-origin. B20 is a blend of 20% biodiesel and 80% mineral diesel, which may partially replace mineral diesel and can be implemented without significant changes in the existing engine hardware. Biodiesel has also shown potential to reduce the problem of CO2 emissions and can partly meet energy needs in rural areas (Wedel, 1999). It needs to be recognized that biodiesel does provide an effective alternative to diesel, but unless it is ensured that emissions from biodiesel (especially toxic pollutants) will be lower, or same as diesel, acceptability of biodiesel as a fuel on a large scale may not be forthcoming. Biodiesel for the present experimental study has been made from rice-bran oil using the process of transesterification using methanol and basic catalyst (KOH). The physical properties of biodiesel (ricebran oil methyl ester), mineral diesel and B20 are shown in Table 1. Out of metals studied in this research (Cr, Ni, Pb, Cd, Na, Al, Mg and Fe) Cr, Ni, Pb, and Cd are toxic metals having a limit of maximum allowable exposure. The acceptable ambient air concentration for Ni is 5 ng m 3, Pb is 1000 ng m 3 and for Cd, it
5587
is 20 ng m 3. Fe, Al, and Mg are generally present in soil in the form of crust metals but excess of these metals in atmosphere is of concern. In addition, profiling of metal content in engine exhaust is necessary to identify and apportion air-pollution sources. The focus of this paper is on comparative assessment and characterization of emissions from traditional diesel and B20 (20% biodiesel blend) fuel exhausts in terms of (i) particulate, (ii) metals in particulate, and (iii) benzene soluble organic fraction (BSOF; a toxicity indicator) in particulate. 2. Materials and methods To characterize the emissions from mineral diesel and B20, a typical medium duty transport engine (Model: MDI 3000 A; Make: Mahindra and Mahindra Ltd., India) was used in present experimental investigations (Fig. 1). This is a four-cylinder, four-stroke, variable-speed, transport engine with direct-injection of fuel. Detailed specifications of the engine are given in Table 2. This engine is installed with an eddy-current dynamometer (Model: ASE-70; Make: Shenck-Avery India Ltd.). The eddy-current dynamometer is equipped with a dynamometer controller capable of loading the engine at the desired speed/load. For collection of particulate for characterization, the engine was operated at loads ranging from idling, 25%, 50%, 75%, to rated engine load at a constant speed of 1800 rpm (rated speed for maximum torque). Particulate samples were collected iso-kinetically using a partial flow dilution tunnel, which was designed and fabricated Fuel Tank
Eddy Current Dynamometer Controller
Air Box
Engine
Table 1 Physical properties of diesel and biodiesel Fuel property
Diesel
B100
B20
Specific gravity at 30 1C Viscosity (cSt) at 40 1C Calorific value (MJ kg 1) Flash point (1C) Sulfur content (ppm)
0.839 3.18 44.8 48 500
0.877 5.30 42.2 265 10
0.847 3.48 44.1 — 400
Exhaust Calorimeter Fig. 1. Mahindra (MDI-3000 A) engine and dynamometer system.
ARTICLE IN PRESS D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
5588 Table 2 Engine specifications Manufacturer
Mahindra & Mahindra Ltd., India
Model No. of cylinders, configuration Combustion system Bore/stroke (mm) Engine displacement (cc) Compression ratio Maximum speed at full load (RPM) Rated power (RPM)
MDI 3000 A Four, in-line Direct injection 88.9/101.6 2520 18:1 2300 40 hp (2300)
layer, which changes the structure, composition and density of the particulate. Partial flow dilution tunnel is used to simulate this near field (o3 m) mixing, condensation and adsorption process in the present study. When the diesel engine is operated, carbonaceous soot particles and high boiling point hydrocarbons are emitted from the tailpipe. These hydrocarbons condense on soot to form particulate matter (PM) after being diluted with preheated air inside the partial flow dilution tunnel. The dilution ratio is typically kept at 10:1. 2.3. Test procedure
Table 3 Sampling details Samples collected for analysis
Load
Sampling duration (min.)
Benzene soluble organic fraction (single sample)
Idle
30
25% 50% 75% 100%
25 25 25 25
Idle 25% 50% 75% 100%
30 25 25 25 25
Metals (duplicate sample)
in-house for this purpose (Dwivedi, 2005), described later. The gaseous emissions were measured using a raw exhaust gas emission analyzer (Model: EXSA1500; Make: HORIBA Ltd., Japan) however this paper discusses the results of particulate and BSOF only. 2.1. Diesel engine and dynamometer system Particulate sample were collected on filter paper (GF/A, 47 mm. Make: Nupore Filtration System, Batch No. 1720704) for analyzing various metals and BSOF. Sampling details are given in Table 3. 2.2. Standard sampling method: dilution tunnel The particulate leaving the exhaust pipe are at relatively high temperature. These particulate and gases which contain hydrocarbons also cool down during the mixing and dilution process with the atmospheric air, and the associated condensation of hydrocarbons takes place on the particulate surface
The first step in the sampling procedure is preparation of filter papers. The filter papers were desiccated for 12 h and then weighed. One filter paper was placed in the filter holder and then the filter assembly was installed in the partial flow dilution tunnel. After running the engine at desired load and speed condition and collecting the particulate through partial flow dilution tunnel for a predetermined period of time, the filter papers were removed from the filter assembly and again kept in desiccators for 12 h and then weighed. Particulate emission was found gravimetrically by measuring difference in weights of filter paper, before and after the particulate sampling. These filter papers were then analyzed for characterization of particulate for metals and BSOF. 2.4. Instruments and measurement systems 2.4.1. Extraction of samples for metal analysis in particulate Extraction of samples for metals analysis was carried out using the hot plate method. In hot acid extraction (USEPA, 1995), the filter strips are placed in a beaker and extracted by refluxing on a hot plate, using 10 mL hydrochloric acid (8%) nitric acid (3%) solution. The digested solution was filtered before analysis. 2.4.2. Sample preparation for analyzing metals in diesel and lubricating oil Sample preparation for metals analysis was carried out using acid digestion of oils for metals analysis by atomic absorption spectrometry (AAS). A representative 0.5 g sample was mixed with 0.5 g of finely ground potassium permanganate and then 1.0 mL of concentrated sulfuric acid was added while stirring. A strong exothermic reaction takes
ARTICLE IN PRESS place. The sample was then treated with 2 mL concentrated nitric acid. Ten mL of concentrated HCl was added and the sample was heated until the reaction was complete and then the metal extract was filtered for further analysis using AAS (USEPA, 2002). 2.4.3. Estimation of BSOF in particulate Analysis for BSOF of the particulate samples was carried out using ASTM test method D 4600-87 (ASTM, 2001). This method is also recommended by the National Institute of Occupational Safety and Health, USA to represent the toxic organic compounds in the particulate. This test method describes the sampling and gravimetric determination of benzene-soluble PM.
Particulates ( mg/m3)
D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
5589
55 45 35 Diesel
25
Biodiesel
15 0%
25%
50%
75%
100%
Engine Load (%) Fig. 2. Variation in particulate emission with engine load.
Table 4 Concentration of various metals in diesel, biodiesel and lubricating oil samples
2.5. Metal analysis using AAS Diesel
AAS (Model: GBC Avanta S, Australia) was used to carry out analysis of samples of diesel, biodiesel and lubricating oil along with particulate samples collected on filter papers. Metals analyzed included Fe, Mg, Cr, Ni, Pb, Zn, Cd, Al, and Na. For analyzing these metals, first standard solutions of known concentration of salts containing these metals were prepared and instrument was calibrated.
Biodiesel
Lubricating oil
247.5 180.5 10.2 15.8 ND ND ND 2.2 1.85
1770 656.55 20.1 27.3 ND ND ND 35.1 1.45
(mg g 1) Na Zn Fe Mg Ni Pb Cd Al Cr
1393.75 152.1 7 9.2 ND 0.93 0.52 1.55 1.3
3. Results and discussion Particulate samples were collected from the compression ignition engine using diesel and B20 and were analyzed for metals and BSOF as per the sampling plan shown in Table 3. This section presents a summary and interpretations of the experimental results for DE and 20% biodiesel exhaust (BDE). 3.1. Particulate One of the objectives of this study was to examine particulate emission in DE and BDE under varying engine load conditions. Fig. 2 shows variations of particulate emission under varying engine loads for diesel and B20 fuel at constant engine speed. Shobokshy (1984) and Sharma et al. (2005) have reported that particulate concentration increases with increased engine load; the same trend is obtained in present study for both the fuels (Fig. 2). However, it is noteworthy that particulate emission is higher in DE
(22–59 mg m 3) than B20 exhaust (17–48 mg m 3). Studies by Sharp (1996) and Wedel (1999) established that a B20 blend (approximately 2% oxygen for RME-20) reduces particulate by approximately 30% compared to particulate in the DE. Fig. 2 shows that rate of increase (with increasing load) of particulate emission is lower for B20 exhaust. This lower increase in particulate concentration in B20 exhaust can be attributed to (i) higher oxygen content in B20 and (ii) lower C/H ratio; in B20 C/H ratio 6.53 compared to diesel 6.82 (Turrio-Baldassarri et al., 2004; Canakci and Van Gerpen, 2001), and (iii) near absence of sulphur and aromatic content of biodiesel. 3.2. Metals in mineral diesel, biodiesel and lubricating oil Table 4 shows measured levels of Fe, Cr, Ni, Pb, Zn, Cd, Na, and Al in diesel, biodiesel (before blending with mineral diesel) and lubricating oil.
ARTICLE IN PRESS D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
5590
The basic metal content in the fuels and lubricating oil are variable e.g. iron is found to be higher in biodiesel, whereas lead and cadmium is not detected. The variable metal contents will have an impact on the metal in the particulate exhaust of diesel and biodiesel. There are other factors like lubricity of the fuel, which affects the wear of fuel injection system and presence of wear metals in particulate to a minor degree.
3.3. Metals in particulate collected from DE and BDE The experimental study showed that concentrations of Fe, Mg, and Na (crust elements) were much higher than those of Cr, Ni, Pb, Zn, Al and Cd (anthropogenic elements) in both the fuels. The similar trend has also been reported by Wang et al. (2000). The results of present experimental investigation for metals in particulate are divided in two sections based on (i) relatively higher concentration in DE and (ii) relatively higher concentration in BDE.
3.4. Metals present in higher concentration in DE Four metals namely lead, cadmium, sodium and nickel are present in higher concentration in the particulate collected from DE compared to BDE (Fig. 3). Metal-wise emissions are discussed below. 3.4.1. Lead It may be noted from Fig. 3 that there are lower emissions of lead in BDE than in DE (0.9–0.30 mg g 1 for DE, 0.7–0.25 mg g 1 for BDE). The analysis of lead concentration in diesel, biodiesel and lubricating oil depicted that it was present only in diesel and was not present in biodiesel and lubricating oil (Table 4). With B20 blend, the emission of lead decreased because absence of lead in biodiesel. 3.4.2. Cadmium There is a slight reduction in emissions of cadmium for BDE compared to DE (0.13–0.027 mg g 1 for DE, 0.12–0.024 mg g 1 for BDE). The analysis of cadmium concentration in diesel, biodiesel and lubricating oil showed that it was present in diesel but was non-detectable in
0.9
Diesel
0.8
B20
Diesel
0.13 Cadmium (mg/g)
Lead (mg/g)
1
0.7 0.6 0.5 0.4
B20 0.09
0.05
0.3 0.2
0.01 0%
25%
50% 75% Engine Load (%)
100%
0%
25%
50% 75% Engine Load (%)
100%
1.4 1.2
Diesel
10
B20
Nickel (mg/g)
Sodium (mg/g)
12
8 6 4 2
1 0.8 0.6 Diesel
0.4
B20
0.2
0 0%
25%
50% 75% Engine Load (%)
100%
0 0%
25%
50% 75% Engine Load (%)
Fig. 3. Concentration of various metals present in higher concentration in particulate from diesel exhaust.
100%
ARTICLE IN PRESS D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
biodiesel and lubricating oil (Table 4). The source of cadmium in DE and BDE is possibly from fuel (mineral diesel) and engine wear. 3.4.3. Sodium It may be noted that sodium shows highest emissions (12–2 mg g 1 for DE, 9–1 mg g 1 for BDE) amongst all crust metals. This is due to the fact that sodium is present in abundance in diesel, biodiesel and lubricating oil (Table 4). Presence of sodium was found highest in lubricating oil followed by diesel and biodiesel. With B20 fuel, the emission of sodium decreased because biodiesel contains lower amount of sodium as compared to mineral diesel. It is seen in Fig. 3 that there is significant difference in sodium content in particulate from DE and BDE at idling. In this study, a small bore (88.9 mm) engine is used. In smaller engines, there is always a possibility of fuel dilution of lubricating oil because of fuel spray impingement on lubricated cylinder walls. This is an undesirable situation however this invariably happens in smaller diesel engines. Fuel lubricity plays an important role in injection system wear and additional lubricity properties of fuel (biodiesel) leads to lower injection system wear (Bijwe et al., 2004). 3.4.4. Nickel Nickel shows emissions in DE and was not detected in case of BDE (1.3–0.7 mg g 1 for DE, Not detected for BDE). As shown in Fig. 3, nickel was either absent or was below detection level in both the fuels and lubricating oil. It can therefore be inferred that whatever nickel is coming out in DE may be from engine wear. With B20, the emission of nickel in particulate is not observed possibly due to self-lubricity property of biodiesel, which results in reduced engine and fuel injection system wear. 3.5. Metals present in higher concentration in BDE Other metals such as iron, aluminum, zinc, chromium and magnesium are found in higher concentration in the particulate collected from BDE compared to DE (Fig. 4). 3.5.1. Iron Fig. 4 presents iron content in particulate drawn from DE and BDE at different engine loads. It may be noted that iron shows highest emission (7–2 mg g 1 for DE, 8–3 mg g 1 for BDE) amongst
5591
all anthropogenic metals. This is due to the fact that iron is present in abundance in diesel, biodiesel and lubricating oil. In addition, iron may also be contributed from engine wear (Agarwal et al., 2003). With B20, the emission of iron is found to be higher because biodiesel contains higher amount of iron compared to mineral diesel (Table 4). 3.5.2. Aluminum It may be noted that aluminum shows higher emissions for biodiesel operation (2.2–0.9 mg g 1 for DE, 2.3–1.5 mg g 1 for BDE; Fig. 4). Aluminum content is higher in biodiesel compared to mineral diesel (Table 4); hence it is reflected in higher content in particulate from BDE. 3.5.3. Zinc It may be noted that zinc concentrations vary from 7.8 to 2.3 mg g 1 for DE and 8.2–3.1 mg g 1 for BDE (Fig. 4). With B20, the emission of zinc increased because biodiesel contains more zinc compared to mineral diesel. There is no significant difference in zinc concentration in particulate from DE and BDE at idling. As load increases the difference seems to be more pronounced because at idling, consumption of lubricating oil is lower for biodiesel due to self-lubricity property of biodiesel. 3.5.4. Chromium It may be noted that chromium shows higher emissions for BDE compared to DE (1.2–0.4 mg g 1 for DE, 1.5–0.6 mg g 1 for BDE; Fig. 4). The analysis of chromium concentration in diesel, biodiesel and lubricating oil shows that its presence in biodiesel is higher than mineral diesel (Table 4). With B20, the emission of chromium increased as compared to diesel because of its higher concentration in fuel itself (Table 4). Difference between the concentration of lubricating oil and biodiesel is not very high so lubricating oil does not play any significant role in emission of chromium. 3.5.5. Magnesium Magnesium concentration varies from 8.7 to 5 mg g 1 for DE and 9.12–6.2 mg g 1 for BDE (Fig. 4). This is due to the fact that magnesium is present in abundance in diesel, biodiesel and lubricating oil (Table 4). It is clear from Fig. 4 that there is not much difference in magnesium content for DE and BDE at idling. As load increases this difference gets pronounced. As load further in-
ARTICLE IN PRESS D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
5592
2.8
8 Iron (mg/g)
Aluminium (mg/g)
Diesel
7
B20
6 5 4 3 2 1
Diesel B20
2.4 2 1.6 1.2 0.8
0%
25%
50%
75%
100%
0%
25%
Engine Load (%)
75%
100%
1.6
8 Diesel B20
7 6
Chromium (mg/g)
ZInc (mg/g)
50% Engine Load (%)
5 4 3
1.4
Diesel B20
1.2 1 0.8
e
0.6 0.4
2
0.2 0%
25%
75%
50%
0%
100%
25%
Magnesium (mg/g)
50%
75%
100%
Engine load (%)
Engine Load(%)
9.5 9 8.5 8 7.5 7 6.5 6 5.5 5
Diesel B20
0%
25%
50%
75%
100%
Engine Load (%) Fig. 4. Concentration of various metals present in higher concentration in particulate from B20 exhaust.
creases, the consumption of lubricating oil also increases. Lubricating oil has a large content of magnesium in organo-metallic additives, which are responsible for the increase in magnesium content in DE and BDE. 3.6. Source of metals in DE and BDE: overall characterization of metal emissions Fig. 5 presents metal levels in particulate from DE and BDE and metals present in fuels. Fe, Mg, Cr, Pb, Al and Cd show a good association between metals in
fuels and metals in exhaust particulate. In B20 fuel, concentration of metals is calculated by mass balance (80% metal content in diesel added with 20% metal content of biodiesel). Zn and Na do not show any such association, it can be inferred from this investigation that sources of Zn and Na in exhaust particulate may be from sources other than fuel. The metal concentration results presented in Figs. 3 and 4 reflect that as load increases, the metal content in particulate gradually decreases. This can be explained by the fact that at higher engine load, combustion takes place at higher
ARTICLE IN PRESS
5 4 3 2 1
12
Mg
Cr
Ni
Pb
Al
7 B20 Biodiesel Exhaust
10
6 5
8
4
6
3 4
2
2
1
0
0 Fe
5593
0 Fe
Cd
Mg
Metals
Cr
Ni
Pb
Al
Metal Content in Diesel (ug/g)
6 Diesel Diesel Exhaust
Metal Content in Particulates (mg/g)
10 9 8 7 6 5 4 3 2 1 0
Metal Content in Diesel (ug/g)
Metal Content in Particulates (mg/g)
D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
Cd
Metals Fig. 5. Sources of metals in diesel and biodiesel exhaust.
60 Diesel Biodiesel
50 BSOF (% w/w)
temperature; leading to improved thermal efficiency. In other words, it is known that the emission of particulate matter is strongly affected by the operating conditions of the engine. In particular, lower engine load/ speed might result in lower thermal efficiency and hence lead to higher particle formation and emission. Brake specific fuel consumption of diesel engine decreases with increasing load/ speed and this is reflected in reduced emission of metals with increasing engine load. It is also reported by Sharma et al. (2005) that overall particulate formation increases in the form of elemental carbon with increase in engine load. Particulate matter emission is higher for higher engine loads because higher amount of fuel is being injected and burnt in the engine. For this reason, the particulate emission from the diesel engine is higher but the corresponding metal content is lower for higher engine load as evident for almost all metals. It must be recognized that at higher engine loads, particulate emission is more in terms of elemental carbon; thereby reducing metal content (mg g 1) in particulate (Ullman, 2004). This is noteworthy that the thermal efficiency of the engine increases with increasing engine load and after a threshold limit (in present study it corresponds to approximately 75–85% of rated load), efficiency starts decreasing. Emission of metals increases at 100% load (compared to 75% load) for almost all metals, due to higher specific brake fuel consumption at full load (Figs. 3 and 4). In view of the above discussions, this can be concluded that metal content in diesel, biodiesel and lubricating oil play an important role in the emission of metals in the engine exhaust and
40 30 20 10 0 0%
25%
50%
75%
100%
Load (%) Fig. 6. Variation of BSOF with engine load.
blending with biodiesel is likely to result in lower emissions of metals. 3.7. BSOF in particulate BSOF has been taken as indicator of organic fraction of particulate which represents the toxic compounds. It can be observed from Fig. 6 that at idling, BSOF was about 48% in DE and 55% in BDE and BSOF gets lowered with increase in engine load. At idling condition, the mineral diesel and lubricating oil undergo partial combustion (pyrolysis) due to low temperature conditions prevailing in the combustion chamber. This leads to higher unburnt hydrocarbon species formation and emissions, which is detected as BSOF (Sharma et al., 2005). Although the exact mechanism for presence of BSOF varies depending on the engine’s operating
ARTICLE IN PRESS 5594
D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595
condition, generally accepted explanation for BSOF in exhaust is that soot particles that are formed in fuel-rich portions of the diesel (spray) collect hydrocarbons through condensation and adsorption (Finch et al., 2002). Higher molecular weight compounds will form small liquid droplets and get adsorbed and deposited on the carbonaceous soot particles. These small droplets may be adsorbed by soot particles or simply be trapped on the filters used to collect particulate. Operating conditions which increase the fraction of the fuel and lubricating oil that remains unburned or partially burned; will increase the BSOF of the particulate. For B20 particulate, this value of BSOF is higher than that of diesel particulate. This is primarily because of lower volatility of constituents of biodiesel; biodiesel can be expected to increase the amount of soluble organic fraction emitted by a compression ignition engine. However, the increased mass consists mostly of unburned esters from the fuel itself (Sharp et al., 2000). Since biodiesel is nontoxic, the increased level of soluble organic fraction may not be hazardous (Finch et al., 2002). However in order to clearly establish the organic toxicity of BDE, more investigations are required through further speciation of exhaust. 4. Conclusions Oxygenated fuel B20 (biodiesel blend) showed superior engine performance in reducing particulate emissions at all operating conditions compared to mineral diesel (particulate in DE; 22–59 mg m 3 and in BDE; 17–48 mg m 3) at constant speed. This may be due to lower sulphur and aromatic content of biodiesel. Along with reduction in particulate matter, there is an overall reduction in metal emissions from the biodiesel exhaust. It was also found that metals in particulate mainly originate from fuel and lubricating oil and in addition some of the metals can originate from engine wear. The reduced metal emissions in BDE are because of (i) inherent lubricity property of biodiesel leading to reduction of emissions of certain metals in BDE due to lower fuel injection system wear and (ii) lower metal levels (Pb and Cd) in biodiesel compared to mineral diesel. There is a net increase in BSOF for B20 fuel as compared to mineral diesel, however biodiesel is nontoxic, hence the increased level of soluble organic fraction may not be viewed as hazardous (Finch et al., 2002). However hazardous nature of
biodiesel exhaust needs further investigations by speciating organic species in biodiesel exhaust. This necessitates that speciation of organic compounds is done for both the fuel exhausts to clearly establish comparative organic toxicity. Nonetheless, this research suggests that overall toxicity of emissions in terms of metals reduces in BDE compared to DE.
References Agarwal, A.K., Bijwe, J., Das, L.M., 2003. Effect of biodiesel utilisation on wear of vital parts in compression ignition engine. Journal of Engineering for Gas Turbine and Power 125, 604–611. ASTM, 2001. D-4600-87: Standard test method for determination of benzene soluble particulate matter in workplace atmosphere. Bijwe, J., Agarwal, A.K., Sharma, A., 2004. Assessment of lubricity of biodiesel blends in reciprocating wear mode. Journal of Fuels & Lubricants, Section 4, 2117–2126 SAE Paper No 2004-01-3068, SAE Transactions 2004. Canakci, M., Van Gerpen, J.H., 2001. Comparison of engine performance and ignition for petroleum diesel fuel, yellow grease biodiesel, and soy bean oil biodiesel. ASAE 0160050, Annual International Meeting, CA, USA. Dwivedi D., 2005. A comparative study of particulate & emission characterization from diesel and biodiesel exhausts. M.Tech. Thesis, submitted to Department of Civil Engineering, IIT Kanpur, India. EPA 420-P-01-001, 2001. Strategies and Issues in Correlating Diesel Fuel Properties with Emissions, July. Finch, G.L., et al., 2002. Effects of subchronic inhalation exposure of rats to emissions from diesel engine burning soybean oil-derived biodiesel fuel. Inhalation Toxicology 14, 1017–1048. Sharma, M., Agarwal, A.K., Bharathi, K.V.L., 2005. Characterization of exhaust particulate from diesel engine. Atmospheric Environment 39, 3023–3028. Sharp, C.A., 1996. Emissions and Lubricity Evaluation of Rapeseed Derived Biodiesel Fuels. Southwest Research Institute, San Antonio, TX. Sharp, C.A., Howell, S.A., Jobe, J., 2000. The effect of biodiesel fuels on transient emissions from modern diesel engines, Part II—unregulated emissions and chemical characterization. SAE Paper No. 2000-01-1968. Shobokshy, M.S.E., 1984. A preliminary analysis of the inhalable particulate lead in the ambient atmosphere of the city of Riyadh, Saudi Arabia. Atmospheric Environment 18 (10), 2125–2130. Turrio-Baldassarri, L., Battistelli Chiara, L., Conti, L., Crebelli, R., Berardis Barbara, D., Iamiceli, A.L., Gambino, M., Iannaccone, S., 2004. Emission comparison of urban bus engine fueled with diesel oil and biodiesel blend. Science of the Total Environment 327, 147–162. Ullman, T.L., 2004. Diesel Emission Testing. Asian Vehicle Emission Control Conference, Beijing, China. USEPA, 1995. Health assessment document for diesel engine exhausts. EPA/600/8-90/057, US Environmental Protection Agency.
ARTICLE IN PRESS D. Dwivedi et al. / Atmospheric Environment 40 (2006) 5586–5595 USEPA, 2002. Health assessment document for diesel engine exhaust. Washington, DC. Wang, W.G., Lyons, D.W., Clark, N.N., Gautam, M., Norton, P.M., 2000. Emissions from heavy trucks fueled by diesel and biodiesel blend without engine modifications. Environmental Science and Technology 34, 933–939.
5595
Wedel, R.von., 1999. Marine Biodiesel in Recreational Boats, second ed. CytoCulture International, Inc., Point Richmond, CA and 22 April, 1999. (Marine Biodiesel and Education Project for San Francisco Bay and Northern California. Prepared for the National Renewable Energy Laboratory US Department of Energy).