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Evaluation of toxic potential of particulates emitted from Jatropha biodiesel fuelled engine
Renewable Energy 99 (2016) 564e572
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Evaluation of toxic potential of particulates emitted from Jatropha biodiesel fuelled engine Avinash Kumar Agarwal a, *, Abhay Shrivastava b, Rajesh Kumar Prasad a a b
Engine Research Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, India Environmental Engineering and Management Program, Department of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 December 2015 Received in revised form 10 May 2016 Accepted 21 July 2016
This study involved a comparative experimental characterization of trace metals and relative toxicity evaluation of particulates emitted by a medium-duty transportation diesel engine using diesel and 20% (v/v) Jatropha biodiesel blend (B20). The engine was operated at 1800 rpm and particulate samples were collected on a filter paper at five loads (0%, 25%, 50%, 75% and 100% rated load) at steady state using a partial flow dilution tunnel, for both diesel and B20. Samples were then analyzed for trace metals, particulate morphology, total organic carbon and benzene soluble organic fraction (BSOF), which is a marker of toxicity. Concentrations of crust elements (Al, Ca, Fe and Mg) in the particulates were relatively higher than that of anthropogenic elements (Cd, Cr, Cu, and Zn) for both test fuels. B20 emitted relatively lower trace metal concentrations in particulates compared to baseline mineral diesel and these concentrations decreased with increasing engine load. Concentrations of gaseous emissions namely CO and THC in biodiesel exhaust were relatively lower than mineral diesel however NOx was relatively higher. Scanning electron microscopy of the particulate samples collected on the filter papers was done along with EDAX analysis for comparison. © 2016 Published by Elsevier Ltd.
Keywords: Biodiesel Trace metals Toxicology Scanning electron microscopy Benzene soluble organic fraction
1. Introduction Diesel vehicles are reliable and highly fuel-efficient compared to their gasoline counterparts and are being mainly used for transportation globally. Diesel engine emissions have attracted attention of researchers and environmentalists alike [1]. Diesel emissions consist of organic as well as inorganic species in gaseous and particulate phases. Carbonaceous particles with adsorbed heavier hydrocarbons are typically termed as diesel particulates matter (DPM). It is well known that composition and toxicity of DPM is strongly influenced by engine operating conditions [2]. The size and chemical composition of DPM may vary with engine design and operating conditions such as engine speed, load, fuel properties, cetane number, spray characteristics, and fuel injection pressure in addition to the lubricating oil used [3,4]. According to a USEPA (2002) assessment, long-term exposure to diesel engine exhaust can cause lung damage, possibly leading to lung cancer. Fine particles from exhaust penetrate deeper into human respiratory tract and can potentially cause serious respiratory diseases [5]. In last
* Corresponding author. E-mail address: [email protected] (A.K. Agarwal). http://dx.doi.org/10.1016/j.renene.2016.07.056 0960-1481/© 2016 Published by Elsevier Ltd.
decade, awareness about the energy-environment nexus has encouraged several researchers to find alternatives to fossil fuels, which may possibly lead to serious health and environmental issues. Amongst various alternative fuels such as biodiesel, ethanol, electricity, hydrogen, methane, LPG, CNG and vegetable oils, which are available today, biodiesel produced from vegetable oils (edible or non-edible) is emerging as most promising and acceptable alternative fuel globally. Vegetable oils derivative fuelled engine may emit higher DPM with strong bacterial mutagenicity [6], however they may have lower unsaturated fatty acids and PAHs emissions [7] therefore lower mutagenic effects [8]. This may be due to the fact that PAH formation is favored in presence of radicals in unsaturated hydrocarbons, compared to saturated hydrocarbons [6]. It may also happen due to decreasing heat of combustion with increasing unsaturation of compounds, leading to less efficient combustion, resulting in higher PAH formation [9]. Biofuels have emerged as one of the ways to address global warming and environmental issues [10] and at the same time, improve engine performance and significantly reduce emissions with minor hardware adjustments. Jatropha curcas is a renewable energy resource, and the seeds contain upto 30e35% (w/w) oil. This non-edible oil contains 21%
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saturated fatty acids and 79% unsaturated fatty acids. In India, Gujarat, Rajasthan, Madhya Pradesh, Maharashtra, Tamilnadu and Andhra Pradesh are the leading states producing Jatropha. Jatropha plants are grown in marginal and poor soils, having pH less than 9 and annual rainfall not more than 600 mm [11]. Its cultivation is simple and requires low capital investment. From second year onwards after planting, the plant starts yielding fruits/oil but maximum yield (3.5e3.75 tons of oil/hectare) can be obtained only from fifth year onwards after planting, upto 40e50 years. Besides higher cetane number, Jatropha oil reduces emission of CO by 44%, sulphates by ~100% and ozone forming potential by < 50%. Vegetable oils can be used directly in IC engines or they can be converted into biodiesel by process of transesterification. Biodiesel is basically mono-alkyl esters of fatty acids, which are derived from vegetable oils or animal fats and can be used either alone (B100) or in blended form such as B20 (20% v/v biodiesel and 80% v/v mineral diesel) in unmodified CI engines. Usage of biodiesel or blends leads to lower emission of DPM. Biodiesel primarily consists of oxygenated hydrocarbons, free from sulphur, aromatic hydrocarbons, and trace metals and has relatively higher cetane number than baseline mineral diesel. Absence of sulphur leads to direct reduction in formation of sulphates in the engine combustion chamber, which are precursors to particulate formation, thereby reducing engine out PM emissions. Agarwal et al. [12] reported that PM number concentration in the exhaust increased with increasing engine load. Higher cetane number of biodiesel results in reduced ignition delay, leading to superior combustion and lower emissions [10,13]. Biodiesel usage results in reduction in CO2 emissions [14,15] however NOx emissions increase with increasing biodiesel concentration in test blends, primarily due to increased availability of additional oxygen in the fuel. Agarwal et al. [16] performed oxidation stability investigations of biodiesel and reported that it can retain oxidation stability even after 4 months from production. Diesel particulates contain several trace metals, which make them hazardous for human health. Hare [17] reported that exhaust gas from a four-stroke, heavy-duty diesel engine contains traces of Si, Cu, and P. Lowenthal et al. [18] reported that trace metals in DPM included Zn, Fe, Ca, P, Ba and La. Total trace metal emissions were > 0.3% of total PM mass, with an emissions rate of 1.65 mg/km. Trace metals emitted by a four-stroke heavy-duty engine included Si, Cu, Ca, Zn and P, whereas a two-stroke engine emitted Pb, Mn, Cr, Zn and Ca. Out of these, Ca, P and Zn are normally present in the lubricating oil [17] as organo-metallic additives. Crebelli et al. [19] reported that BSOF of DPM represent the aromatic compounds and mutagenicity potential of particulates, which mainly depends on BSOF. Sahoo et al. [20] reported that ignition delay of Jatropha biodiesel (JB100) was relatively shorter than baseline mineral diesel. Ong et al. [21] carried out performance and emission investigations on a single cylinder diesel engine using different biodiesel blends (B10, B20, B30 and B50) at full throttle and noticed a significantly reduced emissions of CO2, CO, and smoke opacity, but slightly increased NOx. They reported that B10 yielded superior combustion with relatively higher thermal efficiency compared to baseline mineral diesel. Muralidharan et al. [22] reported that fuel oxygen present in biodiesel helped sustain diffusion combustion phase and reduced dignition delay. Dhar et al. [23] compared performance, emissions and combustion characteristics of Karanja -vis mineral diesel in a 4-cylinder mediumbiodiesel/blends vis-a duty transportation DICI engine and reported that biodiesel/ blends produced lower CO emission but higher NOx emissions at higher engine speeds and loads. Rizwanul et al. [24], and Agarwal et al. [25] also reported that biodiesel blends emitted higher NOx compared to baseline mineral diesel. NOx increased due to higher peak combustion chamber temperature, longer combustion duration and higher oxygen content of biodiesel. Gangwar et al. [3]
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performed parametric study on a common rail direct injection (CRDI) diesel engine operated at different loads using diesel and 20% (v/v) Karanja biodiesel blend (KB20) and reported that BSOF decreased with increasing engine load for both test fuels. Higher BSOF was obtained in case of KB20 compared to diesel, suggesting possibly higher toxicity of biodiesel particulates. Concentration of various trace metals also decreased with increasing engine load. Scanning electron microscopy (SEM) performed on PM collected on glass fiber filters at different engine loads confirmed that particulate morphology was affected by fuel type as well as engine operating conditions. SEM was also employed to examine the crosshatched honing marks on the engine liner [26], which was very crucial for cylinder lubrication. Atomic Absorption Spectroscopy (AAS) is one of the most commonly used techniques for analyzing trace metals, which is traditionally used for carrying out trace metal analysis of diesel, biodiesel and lubricating oil from the engine and particulates collected on the filter papers. This study was carried out to assess the trace metal emissions -vis baseline from 20% (v/v) Jatropha biodiesel blend (B20) vis-a mineral diesel under varying engine loads and assess relative toxic potential of particulates emitted and comparative particulate morphology. This is an essential study before Jatropha biodiesel can be introduced on a large scale in India and in developing countries worldwide. 2. Experimental setup To characterize PM emissions from mineral diesel and biodiesel, a medium-duty engine (Mahindra and Mahindra; MDI 3000) typically used in mid-sized Sports Utility Vehicle (SUV) (Fig. 1) was used in this study. Specifications of the test engine are given in Table 1. This engine was coupled with an eddy-current dynamometer (Schenck-Avery; ASE-70), which was controlled by a dynamometer controller. The dynamometer was capable of loading the engine to the desired speed and load in the entire operating range. For particulates sampling, test engine was operated in the entire load range starting from no load to rated load through 25%, 50%, 75% loads at the rated speed for maximum torque (1800 rpm). Mineral diesel and B20 were used as test fuels. Regulated gaseous emissions were measured using a raw exhaust gas emission analyzer (Horiba; EXSA-1500). Particulate samples were collected isokinetically using a partial flow dilution tunnel on a 47 mm, glass fiber (GF/A) filter paper (Nupore Filtration System; Batch No. 1720704) for analyzing trace metals, BSOF, and particulate morphology. Measurement standards (SAE J1280) recommend direct particulate mass measurement and specify using a dilution tunnel in order to simulate exhaust particulate sampling from ambient air in proximity of the vehicle tail pipe. PM leaving the exhaust tail pipe is at a relatively high temperature and concentration. The temperature of exhaust gas decreases because of atmospheric air dilution, leading to condensation of high boiling point hydrocarbons on the particulate surface. In addition, agglomeration changes the structure and density of PM. Dilution tunnel is used to simulate this near field (< 3 m) atmospheric mixing process. Partial flow dilution tunnel requires a fraction of exhaust gas to be drawn from the exhaust line and then dilute it by pre-conditioned air. Particulates are then collected from this diluted exhaust stream. In the partial flow dilution tunnel, diluted exhaust gas flow rate is maintained constant by controlling the suction pump speed, which is installed downstream of the dilution tunnel and the particulate filter. The exhaust gas is mixed with pre-conditioned dilution air in the dilution tunnel at a particular flow rate, given by Equation (1) and then diluted exhaust gas passes through the particulate filter,
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Fig. 1. Schematic of the experimental setup.
Table 1 Test engine specifications. Manufacturer/model
Mahindra & Mahindra/MDI 3000
No. of cylinders/configuration Combustion system Bore/stroke (mm) Engine displacement (cc) Compression ratio Speed at full load, governed Rated power (RPM) Rated torque (Nm@RPM)
Four/in-line Direct injection 88.9/101.6 2520 18:1 2300 40 hp (2300) 140@1400
where particulates are filtered out.
r¼
Gexh G ¼ exh Gtot Gdil Gsam
(1)
Here, Gexh: Total flow-rate of the engine exhaust Gtot: Flow-rate of the diluted exhaust gas Gdil: Flow-rate of the dilution air r: Split ratio Gsam: Flow-rate of the exhaust gas to be sampled
3. Results and discussion In this section, experimental results of the engine experiments are discussed.
The dilution ratio can be calculated by using Equation (2):
dilution ratioðrÞ ¼
½CO2 total exhaust ½CO2 atmosphere ½CO2 engine exhaust ½CO2 atmosphere
exhaust gas-flow rate (Gexh) simultaneously. In this study, the dilution ratio was maintained at 10:1. For particulate sampling, the filter papers were desiccated initially for removing the moisture, if any, for 12 h and then weighed again. One filter paper was placed in the filter holder and then the filter assembly was assembled with the partial flow dilution tunnel. After running the engine at a desired load and speed condition and collecting the particulates in the partial flow dilution tunnel for a predetermined period of time, the filter paper with particulate sample collected on it, was removed from the filter assembly and again kept in a desiccator for moisture removal. The filter papers with particulate samples collected in the entire study were analyzed simultaneously for trace metals content, and BSOF. For trace metal analysis, Atomic Absorption Spectroscope (AAS) (Avanta S; GBC) was used for analyzing diesel, biodiesel and lubricating oil along with particulate samples collected on the filter papers. BSOF analysis of particulates was carried out as per ASTM D 4600-87 (ASTM, 2001).
3.1. Particulate mass
(2)
The CO2 concentrations can be measured in both, engine exhaust and the diluted sample in order to calculate dilution ratio using Equation (2). To keep the split ratio (r) constant, while maintaining the diluted exhaust gas flow-rate (Gtot) constant, it is necessary to control both the dilution air flow-rate (Gdil) and
Fig. 2 shows the variation in particulate mass emissions under different engine operating conditions using mineral diesel and B20. A known flow rate of exhaust gas was passed through the filter paper in the dilution tunnel for a fixed period of time and the weight of particulates collected was used for generating this data (Fig. 2). Particulates measured in the exhaust from both test fuels increased with increasing engine load however at any particular engine load, particulate emissions from mineral diesel were
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Fig. 2. Variations in particulate mass collected on the filter paper with engine load.
relatively higher compared to B20. Biodiesel has inherently superior lubricity [27] compared to baseline mineral diesel therefore it offers lower frictional losses in the engine powertrain. Bulk modulus of compressibility of biodiesel is relatively higher than baseline mineral diesel, which results in marginally advanced fuel injection compared to mineral diesel. In addition, there is fuel oxygen present in biodiesel molecules, which is available for participation in combustion. This improves combustion efficiency [28] and reduces particulate formation. Biodiesel has relatively shorter ignition delay compared to mineral diesel, because it has relatively higher cetane number. Earlier start of combustion of biodiesel droplets results in relatively higher combustion temperature in the cylinder therefore the particulate formed re-burn, before they are emitted in the tail pipe of the engine/vehicle. All these factors lead to lower particulate formation from B20 compared to mineral diesel, under identical engine operating conditions (Fig. 2).
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well as both test fuels and lubricating oil. Particulate sampling was done in triplicate at five engine loads: 0%, 25%, 50%, 75% and 100% at 1800 rpm. SEM analysis was done for the particulate collected on filter paper at rated load using mineral diesel and B20. Trace metal concentrations in diesel, B20 and lubricating oil are shown in Fig. 3. It can be observed from Fig. 3 that Pb, Mg, and Fe were relatively lower in biodiesel compared to baseline mineral diesel. Lubricating oil contained an order of magnitude higher concentrations of Cr, Mg, Ca, and Zn, than mineral diesel and biodiesel. Concentrations of toxic trace metals such as Pb, Cd etc. were lower in biodiesel compared to mineral diesel. Concentration of crustal trace metals such as Ca, Fe, and Mg were higher than that of anthropogenic trace metals such as Cr, Cd, and Pb. Ni was below detectable limits in all samples. Extraction of trace metal elements from particulate samples collected on the filter paper was done using hot plate extraction method. In hot plate extraction method, filter paper strips laden with particulates were placed in a beaker and trace metals were extracted by refluxing on a hot plate, using 10 ml conc. hydrochloric acid (8%) and conc. nitric acid (3%). HPLC grade acid was used, which was free from trace metal contaminants. The extracts were then diluted with mili-Q water to make up 100 ml solutions. The samples were then analyzed in AAS after calibrating the equipment. Concentration of each trace metal was detected three times and an average of three reading was used for calculating the trace metal concentration in the particulates. This data of trace metal concentration in particulates drawn from mineral diesel and B20 fuelled engine at different engine loads is shown in Fig. 4. Each trace metal is discussed separately in the following paragraphs. 3.2.1. Iron Fe concentration was relatively lower in biodiesel particulates compared to mineral diesel particulates. This was mainly because Fe trace concentration was also lower in biodiesel compared to mineral diesel (Fig. 3).
3.2. Trace metals Trace metal elements such as Pb, Cr, Cd, Ni, Mg, Fe, Na, Ca, Zn and Al were analyzed in the particulates collected on the filter papers as
3.2.2. Aluminum Aluminum occurs naturally in soil, water, and air. High levels in the environment can be due to mining and processing of
Fig. 3. Trace metals in mineral diesel, biodiesel and lubricating oil.
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aluminum ores or production of aluminum, alloys, and compounds. Small amount of aluminum is released into the environment from coal-fired power plants and incinerators. Workers who breathe large amounts of aluminum dust can have lung problems, such as coughing or changes that show up in chest X-rays. Al concentration decreased with increased engine load for both fuels. Al traces in particulates were higher compared to Ca, Mg, and Fe. Al traces originate from the wear of aluminum containing engine components such as piston. The trend of Al traces in particulates suggested that B20 fuelled engine underwent relatively lower wear of aluminum containing components compared to mineral diesel fuelled engine and this was because of the inherent lubricity properties of biodiesel. 3.2.3. Lead Lead is one of the most toxic trace metals, whose widespread use affects multiple systems in human body, including its neurological, hematological, gastrointestinal, cardiovascular, and renal development. People get exposed to lead by inhalation of lead containing compounds, which are generated during smelting, unorganized recycling, use of leaded gasoline and leaded paint, leadcontaminated dust typically originating from bearings, water (from leaded pipes), and food (from lead-glazed or lead-soldered containers) [29]. Children are particularly vulnerable to severe neurotoxic effects of lead, and even relatively low levels of exposure can cause serious health effects and in some cases irreversible neurological damage among children. In developing countries, efforts are being made to eliminate non-essential usage of lead such as use of lead in gasoline and paint, ensuring safe recycling of leadcontaining waste, educating public about importance of safe disposal of lead-acid batteries and computers, and monitoring of blood lead levels in children, women of child-bearing age and workers. In these experiments, Pb concentration in the diesel exhaust particulate did not show any trend for these test fuels. Trace concentration of lead in biodiesel particulates was relatively higher at low loads however it was relatively higher in mineral diesel particulates at higher loads. Lead concentration in biodiesel was relatively lower than mineral diesel (Fig. 3).
3.2.4. Magnesium Mg containing dust may irritate mucous membranes or upper respiratory tract. Exposure to magnesium oxide fumes subsequent to burning, welding or molten metal work can result in metal fume fever, which has temporary symptoms such as fever, chills, nausea, vomiting & muscle pain [30]. Mg concentration in B20 particulates at no load was 12.22 mg/g whereas it was 9.50 mg/g in mineral diesel particulates. B20 particulates showed decreasing trend with increasing load because of additional lubricity offered by biodiesel. However in case of mineral diesel particulates, Mg concentration remained almost constant with load. Mg concentration in B20 particulates was lower than mineral diesel at most operating conditions, because biodiesel had lower concentration of Mg traces compared to mineral diesel (Fig. 3). 3.2.5. Chromium Chromium is one of the most widely used metals in industry, such as in steel production, alloy preparation, wood preservation, leather tanning, paints, pigments, metal plating, tanning, electroplating, steel manufacture and other industrial applications [31]. Cr trace concentration in B20 particulate was slightly higher compared to mineral diesel particulates for most loads. This was linked to higher Cr trace concentration in B20 than mineral diesel (Fig. 3). At higher loads, concentration of chromium was relatively lower in B20 particulates because diesel had relatively inferior lubricity properties compared to biodiesel therefore the piston rings, which have Cr coating, wear faster in case of diesel. Thus the Cr traces originating from wear debris at full load were observed in diesel particulates. 3.2.6. Sodium In transesterification process, sodium hydroxide is used as a catalyst. Sodium is also a component of sodium chloride (NaCl), therefore is a very important compound found everywhere in the living environment. Contact of sodium with water, including perspiration causes formation of sodium hydroxide fumes, which are highly irritating to the skin, eyes, nose and throat. This may cause sneezing and coughing. Na trace concentration in B20 particulates was slightly higher
Fig. 4. Trace metals concentrations in exhaust particulates from mineral diesel and B20 fuelled engine.
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than mineral diesel particulates at all loads. Slightly higher concentration of Na in B20 particulates may be due to presence of traces of catalyst (NaOH) in biodiesel, which remains from the transesterification process. Na trace concentration in particulate did not vary significantly with engine load. 3.2.7. Zinc Zinc is found in the air, soil, and water and is present in all foods. Most zinc enters the environment from mining, purifying of zinc, lead, and cadmium ores, steel production, coal burning, and burning of wastes. Inhaling large amounts of zinc (as zinc dust or fumes from smelting or welding) can cause a specific short-term disease called metal fume fever, which is generally reversible once exposure to zinc ceases [30]. Zn trace concentration in exhaust particulates decreased with load for both test fuels. Zn was largely present in the lubricating oil as anti-wear additive and some of these additives deplete with time. Zn concentration in diesel particulate was relatively higher at all engine loads compared to B20 and it suggested that mineral diesel depleted the additives in the lubricating oil at a faster rate compared to B20. 3.2.8. Calcium The distribution of calcium is very wide; it is found in almost every terrestrial area of the world. It is contained in the soft tissue, and in the fluids within the plant tissues. The use of more than 2.5 g of calcium per day without a medical necessity can lead to development of kidney stones and sclerosis of kidneys and blood vessels [30]. Ca trace concentration in mineral diesel particulates was relatively higher for most engine operating conditions. Ca trace concentration generally decreased with increasing engine load. Ca trace concentration was higher in biodiesel compared to mineral diesel. It seems that the lubricating oil consumption played a major role in Ca trace concentration in particulates. 3.3. Benzene soluble organic fraction BSOF analysis for particulate samples was carried out using ASTM D 4600-87 (2001). This method is recommended by National Institute of Occupational Safety and Health (NIOSH), USA to evaluate toxic organic compounds in particulate samples. This test method describes sampling and gravimetric determination of BSOF of the particulate matter. The soluble organic fraction (SOF) of diesel particulates originate primarily from unburned fuel, lubricating oil, and is essentially a product of partial combustion/pyrolysis. Majority of SOF is classified as unresolvable complex material. In the high temperature exhaust system, these SOF compounds remain in gas phase, however upon cooling and dilution while they move downstream in the exhaust system, some of these less volatile organic species adsorb on to the surfaces of the particulates, forming SOF portion of the DPM. The USEPA's Mobile Source Air Toxic (MSAT) list includes seven PAHs namely benzo(a)antracene, benzo(b)fluoranthene, benzo(k)fluoranthene, chrysene, benzo(a)pyrene, indeno(1,2,3-cd) pyrene, and 7,12-dimethylbenz(a)anthracene [32]. Many of these SOF compounds in DPM have shown mutagenic activity in a variety of assay systems, e.g. ethylene, benzene, 1,3butadiene, acrolein and several PAHs. Particle bound PAHs and nitro-PAHs have been the focus of several mutagenic studies, both in bacteria and in mammalian cell systems. PAHs (> 4 rings, e.g., pyrene, benzo[a]pyrene) forming nitro-PAHs and nitro-PAH lactones in atmospheric reactions are major contributors of carcinogens amongst combustion generated toxins. Many nitro-PAHs (> 3 rings, e.g., nitropyrenes) are potent mutagens and carcinogens and
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some reaction products (hydroxylated-nitro derivatives) are mutagenic in bacteria. Nitro-PAHs with more than two rings stay in particle phase (USEPA, 2002). In mammals, PAHs are activated by the enzyme cytochrome P450 dependent monooxygenase, to form dihydrodiol epoxides. These epoxides are capable of forming adducts with DNA, which lead to induction of tumors. Known cancer causing PAHs are pyrene, benzo[a]anthracene, chrysene, benzo[b] fluoranthene, benzo[a]pyrene, benzo[ghi]perylene, indeno[1,2,3cd]pyrene and dibenzo[a,h]anthracene [33]. BSOF is accepted as an indicator of organic fraction of particulates [34]. Organic fraction in particulates is largely composed of toxic compounds therefore BSOF is accepted as a marker of toxicity of diesel exhaust particulates. Particulate sampling was done at five different engine loads: 0%, 25%, 50%, 75% and 100% at 1800 rpm for BSOF determination using mineral diesel and B20 (Fig. 5). Both test fuels showed the same trend i.e. BSOF decreased with increased engine load. This was primarily due to higher in-cylinder temperatures prevailing at higher engine loads, which destroyed BSOF species formed during incomplete combustion in the fuel-rich zones of the combustion chamber. BSOF was found to be higher for B20 particulates at all operating points, compared to mineral diesel particulates, suggesting possibly higher toxicity of particulates emitted by a biodiesel fuelled engine. At idling, BSOF was ~52% in mineral diesel particulates and ~61% in B20 particulates however with increased engine load, BSOF decreased for both test fuels. This phenomenon can be explained by the fact that fuels (Mineral diesel and B20) and lubricating oil undergo partial combustion due to relatively lower temperatures prevailing in the combustion chamber, particularly at low loads, resulting in formation of higher unburned hydrocarbon species, which formed BSOF. Several researchers [35e41] reported that soot is formed in the fuel-rich regions of diesel spray, which collect hydrocarbons through condensation and adsorption process in later stages. Since biodiesel has lower volatility, proportion of BSOF detected in biodiesel exhaust particulates is also relatively higher. Sharp et al. [38] reported that higher BSOF in biodiesel particulates mostly consisted of unburnt esters from biodiesel. It is also possible that higher molecular weight compounds form smaller liquid droplets and adhere to soot. These small liquid droplets may either agglomerate with other droplets or condense on the soot particles or simply get trapped by the filters. 3.4. Regulated gaseous emissions Measurements were done to detect regulated gaseous emission species such as CO, CO2, THC and NOx (Fig. 6).
Fig. 5. BSOF in the exhaust particulates from mineral diesel and B20 fuelled engine.
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3.4.1. THC Both B20 and mineral diesel followed the same HC emission trend with varying engine load. THC emission decreased with increasing engine load. Lower THC emissions from B20 indicated that oxygenated fuel (biodiesel) emitted lower THC during combustion due to presence of fuel oxygen, which led to more complete combustion [13]. 3.4.2. NOx Both nitric oxide (NO) and nitrogen dioxide (NO2) are grouped together and termed as NOx. NO is the dominant oxide of nitrogen formed in engine combustion chamber and its emission mainly depends on engine operating condition and the type of fuel used. Due to presence of oxygen in biodiesel fuel molecules, superior combustion efficiency was obtained because of higher peak combustion chamber temperature. As a consequence, NO formation in the exhaust was relatively higher in B20 engine compared to mineral diesel fuelled engine. At low loads, NO concentration was slightly lower in biodiesel exhaust than mineral diesel exhaust because biodiesel has higher cetane number and superior autoignition characteristics, which led to shorter ignition delay and lower premixed combustion, resulting in slightly lower NOx formation at low loads [42e44].
Fig. 6. Regulated emissions from mineral diesel and B20 fuelled engine.
3.4.3. CO2 CO2 concentration increased with increasing engine load. Both, B20 and mineral diesel showed same trend throughout the engine operating range. CO2 emission from B20 fuelled engine was slightly lower than mineral diesel fuelled engine. This was primarily due to biodiesel being an oxygenated fuel, which was the main reason for its relatively more efficient combustion, resulting in lower CO2 emissions.
Fig. 7. Morphology of B20 and mineral diesel particulates.
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3.4.4. CO CO concentration varied with varying load. Both, B20 and mineral diesel showed same trend throughout the engine operating range. Initially at no load, CO emission was higher but it decreased with increasing load upto 75% rated load for both fuels. Thereafter CO emission increased significantly at full load. CO concentration from mineral diesel engine was slightly higher compared to B20 at all engine loads. Agarwal et al. [25] also reported similar trend for Karanja oil blended with mineral diesel. Inherent oxygen present in biodiesel helped in obtaining relatively more complete combustion, resulting in lower CO emission from B20 at all operating conditions [28]. 3.5. Scanning electron microscopy Morphology of particulates from B20 and diesel fuelled engines at three different engine loads (0%, 50% and 100% rated load) is shown in SEM micrographs (Fig. 7). SEM images were taken at 2000 magnification for all particulate samples collected on the filter papers using a partial flow dilution tunnel. SEM analysis showed that particulate surface morphology changed with engine load. Layered and plate like structures were observed at lower loads, whereas bigger size particles appeared at higher load conditions. From Fig. 7, it is clear that number concentration of particulates seems higher for mineral diesel, while size of particulates seems larger for biodiesel. The sizes of particulates obtained using SEM were sub-micron for biodiesel as well as mineral diesel. 3.6. Energy dispersion X-ray The filter paper containing particulates emitted by mineral diesel and B20 fuelled engine was subjected to energy dispersion Xray (EDAX) analysis in order to detect the elemental composition of particulates collected on the filter paper.. A comparative analysis of trace metals of particulates from diesel and B20 engines using EDAX is shown in Fig. 8. At no load, biodiesel particulates contained higher elemental concentration of trace metals except Pb. However at intermediate loads, Ni, Cr and Ca trace concentrations were lower in biodiesel particulates. At full load, trace concentrations of Pb, Cu, Ni, and Cr were relatively lower in B20 particulates compared to mineral diesel particulates. Cr concentration using both AAS and EDAX methods showed similar trend. In both methods, Cr concentration was found to be -vis relatively lower at high engine loads in case of biodiesel, vis-a baseline mineral diesel. Al and Ca traces concentrations decreased with increasing engine load using AAS method however opposite trend was seen with EDX technique. There were some disagreements in the trace metals in the particulates using AAS and EDAX. However AAS technique was more precise and superior to EDAX. 4. Conclusions Experimental investigations were performed to determine trace metal concentrations and benzene soluble organic fraction (BSOF) of particulates in the exhaust of a medium-duty SUV diesel engine, fuelled by mineral diesel and 20% (v/v) blend of Jatropha biodiesel (B20). Main conclusions from this experimental study are as follows: Biodiesel derived from Jatropha curcus oil showed superior engine performance and emitted lower particulate mass at all engine operating conditions, compared to baseline mineral diesel.
Fig. 8. Trace metal analysis of mineral diesel and B20 particulates using EDAX.
Concentrations of various trace metals in particulates emitted by mineral diesel and B20 were determined using atomic absorption spectroscopy (AAS). Trace metal concentrations in the particulates were lower for B20 compared to baseline mineral diesel at high loads, possibly because of relatively higher lubricating oil consumed by mineral diesel compared to B20. Crust metals (Ca, Fe, Mg and Al) were generally in higher trace concentrations in particulates emitted by mineral diesel engine. However anthropogenic metals (Pb, Cr, Zn, Na) were generally in higher trace concentrations in B20 at lower engine loads and in mineral diesel at higher engine loads. Benzene soluble organic fraction (BSOF), which is a marker of particulate toxicity, was higher in biodiesel particulates at all loads, indicating possibly higher toxicity of biodiesel particulates. Gaseous emissions (CO, HC and CO2, except NOx) were lower from B20 engine compared to mineral diesel engine. NOx emissions were relatively higher from B20 because biodiesel has fuel oxygen. Morphology of exhaust particulates was analyzed by SEM. Particulate size was found to be larger for particulates emitted by biodiesel fuelled engine. Overall, biodiesel particulates showed higher toxicity than mineral diesel however their trace metal concentrations were comparable to mineral diesel trace metal concentrations. GHG
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Evaluation of toxic potential of particulates emitted from Jatropha biodiesel fuelled engine Avinash Kumar Agarwal a, *, Abhay Shrivastava b, Rajesh Kumar Prasad a a b
Engine Research Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, India Environmental Engineering and Management Program, Department of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 December 2015 Received in revised form 10 May 2016 Accepted 21 July 2016
This study involved a comparative experimental characterization of trace metals and relative toxicity evaluation of particulates emitted by a medium-duty transportation diesel engine using diesel and 20% (v/v) Jatropha biodiesel blend (B20). The engine was operated at 1800 rpm and particulate samples were collected on a filter paper at five loads (0%, 25%, 50%, 75% and 100% rated load) at steady state using a partial flow dilution tunnel, for both diesel and B20. Samples were then analyzed for trace metals, particulate morphology, total organic carbon and benzene soluble organic fraction (BSOF), which is a marker of toxicity. Concentrations of crust elements (Al, Ca, Fe and Mg) in the particulates were relatively higher than that of anthropogenic elements (Cd, Cr, Cu, and Zn) for both test fuels. B20 emitted relatively lower trace metal concentrations in particulates compared to baseline mineral diesel and these concentrations decreased with increasing engine load. Concentrations of gaseous emissions namely CO and THC in biodiesel exhaust were relatively lower than mineral diesel however NOx was relatively higher. Scanning electron microscopy of the particulate samples collected on the filter papers was done along with EDAX analysis for comparison. © 2016 Published by Elsevier Ltd.
Keywords: Biodiesel Trace metals Toxicology Scanning electron microscopy Benzene soluble organic fraction
1. Introduction Diesel vehicles are reliable and highly fuel-efficient compared to their gasoline counterparts and are being mainly used for transportation globally. Diesel engine emissions have attracted attention of researchers and environmentalists alike [1]. Diesel emissions consist of organic as well as inorganic species in gaseous and particulate phases. Carbonaceous particles with adsorbed heavier hydrocarbons are typically termed as diesel particulates matter (DPM). It is well known that composition and toxicity of DPM is strongly influenced by engine operating conditions [2]. The size and chemical composition of DPM may vary with engine design and operating conditions such as engine speed, load, fuel properties, cetane number, spray characteristics, and fuel injection pressure in addition to the lubricating oil used [3,4]. According to a USEPA (2002) assessment, long-term exposure to diesel engine exhaust can cause lung damage, possibly leading to lung cancer. Fine particles from exhaust penetrate deeper into human respiratory tract and can potentially cause serious respiratory diseases [5]. In last
* Corresponding author. E-mail address: [email protected] (A.K. Agarwal). http://dx.doi.org/10.1016/j.renene.2016.07.056 0960-1481/© 2016 Published by Elsevier Ltd.
decade, awareness about the energy-environment nexus has encouraged several researchers to find alternatives to fossil fuels, which may possibly lead to serious health and environmental issues. Amongst various alternative fuels such as biodiesel, ethanol, electricity, hydrogen, methane, LPG, CNG and vegetable oils, which are available today, biodiesel produced from vegetable oils (edible or non-edible) is emerging as most promising and acceptable alternative fuel globally. Vegetable oils derivative fuelled engine may emit higher DPM with strong bacterial mutagenicity [6], however they may have lower unsaturated fatty acids and PAHs emissions [7] therefore lower mutagenic effects [8]. This may be due to the fact that PAH formation is favored in presence of radicals in unsaturated hydrocarbons, compared to saturated hydrocarbons [6]. It may also happen due to decreasing heat of combustion with increasing unsaturation of compounds, leading to less efficient combustion, resulting in higher PAH formation [9]. Biofuels have emerged as one of the ways to address global warming and environmental issues [10] and at the same time, improve engine performance and significantly reduce emissions with minor hardware adjustments. Jatropha curcas is a renewable energy resource, and the seeds contain upto 30e35% (w/w) oil. This non-edible oil contains 21%
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saturated fatty acids and 79% unsaturated fatty acids. In India, Gujarat, Rajasthan, Madhya Pradesh, Maharashtra, Tamilnadu and Andhra Pradesh are the leading states producing Jatropha. Jatropha plants are grown in marginal and poor soils, having pH less than 9 and annual rainfall not more than 600 mm [11]. Its cultivation is simple and requires low capital investment. From second year onwards after planting, the plant starts yielding fruits/oil but maximum yield (3.5e3.75 tons of oil/hectare) can be obtained only from fifth year onwards after planting, upto 40e50 years. Besides higher cetane number, Jatropha oil reduces emission of CO by 44%, sulphates by ~100% and ozone forming potential by < 50%. Vegetable oils can be used directly in IC engines or they can be converted into biodiesel by process of transesterification. Biodiesel is basically mono-alkyl esters of fatty acids, which are derived from vegetable oils or animal fats and can be used either alone (B100) or in blended form such as B20 (20% v/v biodiesel and 80% v/v mineral diesel) in unmodified CI engines. Usage of biodiesel or blends leads to lower emission of DPM. Biodiesel primarily consists of oxygenated hydrocarbons, free from sulphur, aromatic hydrocarbons, and trace metals and has relatively higher cetane number than baseline mineral diesel. Absence of sulphur leads to direct reduction in formation of sulphates in the engine combustion chamber, which are precursors to particulate formation, thereby reducing engine out PM emissions. Agarwal et al. [12] reported that PM number concentration in the exhaust increased with increasing engine load. Higher cetane number of biodiesel results in reduced ignition delay, leading to superior combustion and lower emissions [10,13]. Biodiesel usage results in reduction in CO2 emissions [14,15] however NOx emissions increase with increasing biodiesel concentration in test blends, primarily due to increased availability of additional oxygen in the fuel. Agarwal et al. [16] performed oxidation stability investigations of biodiesel and reported that it can retain oxidation stability even after 4 months from production. Diesel particulates contain several trace metals, which make them hazardous for human health. Hare [17] reported that exhaust gas from a four-stroke, heavy-duty diesel engine contains traces of Si, Cu, and P. Lowenthal et al. [18] reported that trace metals in DPM included Zn, Fe, Ca, P, Ba and La. Total trace metal emissions were > 0.3% of total PM mass, with an emissions rate of 1.65 mg/km. Trace metals emitted by a four-stroke heavy-duty engine included Si, Cu, Ca, Zn and P, whereas a two-stroke engine emitted Pb, Mn, Cr, Zn and Ca. Out of these, Ca, P and Zn are normally present in the lubricating oil [17] as organo-metallic additives. Crebelli et al. [19] reported that BSOF of DPM represent the aromatic compounds and mutagenicity potential of particulates, which mainly depends on BSOF. Sahoo et al. [20] reported that ignition delay of Jatropha biodiesel (JB100) was relatively shorter than baseline mineral diesel. Ong et al. [21] carried out performance and emission investigations on a single cylinder diesel engine using different biodiesel blends (B10, B20, B30 and B50) at full throttle and noticed a significantly reduced emissions of CO2, CO, and smoke opacity, but slightly increased NOx. They reported that B10 yielded superior combustion with relatively higher thermal efficiency compared to baseline mineral diesel. Muralidharan et al. [22] reported that fuel oxygen present in biodiesel helped sustain diffusion combustion phase and reduced dignition delay. Dhar et al. [23] compared performance, emissions and combustion characteristics of Karanja -vis mineral diesel in a 4-cylinder mediumbiodiesel/blends vis-a duty transportation DICI engine and reported that biodiesel/ blends produced lower CO emission but higher NOx emissions at higher engine speeds and loads. Rizwanul et al. [24], and Agarwal et al. [25] also reported that biodiesel blends emitted higher NOx compared to baseline mineral diesel. NOx increased due to higher peak combustion chamber temperature, longer combustion duration and higher oxygen content of biodiesel. Gangwar et al. [3]
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performed parametric study on a common rail direct injection (CRDI) diesel engine operated at different loads using diesel and 20% (v/v) Karanja biodiesel blend (KB20) and reported that BSOF decreased with increasing engine load for both test fuels. Higher BSOF was obtained in case of KB20 compared to diesel, suggesting possibly higher toxicity of biodiesel particulates. Concentration of various trace metals also decreased with increasing engine load. Scanning electron microscopy (SEM) performed on PM collected on glass fiber filters at different engine loads confirmed that particulate morphology was affected by fuel type as well as engine operating conditions. SEM was also employed to examine the crosshatched honing marks on the engine liner [26], which was very crucial for cylinder lubrication. Atomic Absorption Spectroscopy (AAS) is one of the most commonly used techniques for analyzing trace metals, which is traditionally used for carrying out trace metal analysis of diesel, biodiesel and lubricating oil from the engine and particulates collected on the filter papers. This study was carried out to assess the trace metal emissions -vis baseline from 20% (v/v) Jatropha biodiesel blend (B20) vis-a mineral diesel under varying engine loads and assess relative toxic potential of particulates emitted and comparative particulate morphology. This is an essential study before Jatropha biodiesel can be introduced on a large scale in India and in developing countries worldwide. 2. Experimental setup To characterize PM emissions from mineral diesel and biodiesel, a medium-duty engine (Mahindra and Mahindra; MDI 3000) typically used in mid-sized Sports Utility Vehicle (SUV) (Fig. 1) was used in this study. Specifications of the test engine are given in Table 1. This engine was coupled with an eddy-current dynamometer (Schenck-Avery; ASE-70), which was controlled by a dynamometer controller. The dynamometer was capable of loading the engine to the desired speed and load in the entire operating range. For particulates sampling, test engine was operated in the entire load range starting from no load to rated load through 25%, 50%, 75% loads at the rated speed for maximum torque (1800 rpm). Mineral diesel and B20 were used as test fuels. Regulated gaseous emissions were measured using a raw exhaust gas emission analyzer (Horiba; EXSA-1500). Particulate samples were collected isokinetically using a partial flow dilution tunnel on a 47 mm, glass fiber (GF/A) filter paper (Nupore Filtration System; Batch No. 1720704) for analyzing trace metals, BSOF, and particulate morphology. Measurement standards (SAE J1280) recommend direct particulate mass measurement and specify using a dilution tunnel in order to simulate exhaust particulate sampling from ambient air in proximity of the vehicle tail pipe. PM leaving the exhaust tail pipe is at a relatively high temperature and concentration. The temperature of exhaust gas decreases because of atmospheric air dilution, leading to condensation of high boiling point hydrocarbons on the particulate surface. In addition, agglomeration changes the structure and density of PM. Dilution tunnel is used to simulate this near field (< 3 m) atmospheric mixing process. Partial flow dilution tunnel requires a fraction of exhaust gas to be drawn from the exhaust line and then dilute it by pre-conditioned air. Particulates are then collected from this diluted exhaust stream. In the partial flow dilution tunnel, diluted exhaust gas flow rate is maintained constant by controlling the suction pump speed, which is installed downstream of the dilution tunnel and the particulate filter. The exhaust gas is mixed with pre-conditioned dilution air in the dilution tunnel at a particular flow rate, given by Equation (1) and then diluted exhaust gas passes through the particulate filter,
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Fig. 1. Schematic of the experimental setup.
Table 1 Test engine specifications. Manufacturer/model
Mahindra & Mahindra/MDI 3000
No. of cylinders/configuration Combustion system Bore/stroke (mm) Engine displacement (cc) Compression ratio Speed at full load, governed Rated power (RPM) Rated torque (Nm@RPM)
Four/in-line Direct injection 88.9/101.6 2520 18:1 2300 40 hp (2300) 140@1400
where particulates are filtered out.
r¼
Gexh G ¼ exh Gtot Gdil Gsam
(1)
Here, Gexh: Total flow-rate of the engine exhaust Gtot: Flow-rate of the diluted exhaust gas Gdil: Flow-rate of the dilution air r: Split ratio Gsam: Flow-rate of the exhaust gas to be sampled
3. Results and discussion In this section, experimental results of the engine experiments are discussed.
The dilution ratio can be calculated by using Equation (2):
dilution ratioðrÞ ¼
½CO2 total exhaust ½CO2 atmosphere ½CO2 engine exhaust ½CO2 atmosphere
exhaust gas-flow rate (Gexh) simultaneously. In this study, the dilution ratio was maintained at 10:1. For particulate sampling, the filter papers were desiccated initially for removing the moisture, if any, for 12 h and then weighed again. One filter paper was placed in the filter holder and then the filter assembly was assembled with the partial flow dilution tunnel. After running the engine at a desired load and speed condition and collecting the particulates in the partial flow dilution tunnel for a predetermined period of time, the filter paper with particulate sample collected on it, was removed from the filter assembly and again kept in a desiccator for moisture removal. The filter papers with particulate samples collected in the entire study were analyzed simultaneously for trace metals content, and BSOF. For trace metal analysis, Atomic Absorption Spectroscope (AAS) (Avanta S; GBC) was used for analyzing diesel, biodiesel and lubricating oil along with particulate samples collected on the filter papers. BSOF analysis of particulates was carried out as per ASTM D 4600-87 (ASTM, 2001).
3.1. Particulate mass
(2)
The CO2 concentrations can be measured in both, engine exhaust and the diluted sample in order to calculate dilution ratio using Equation (2). To keep the split ratio (r) constant, while maintaining the diluted exhaust gas flow-rate (Gtot) constant, it is necessary to control both the dilution air flow-rate (Gdil) and
Fig. 2 shows the variation in particulate mass emissions under different engine operating conditions using mineral diesel and B20. A known flow rate of exhaust gas was passed through the filter paper in the dilution tunnel for a fixed period of time and the weight of particulates collected was used for generating this data (Fig. 2). Particulates measured in the exhaust from both test fuels increased with increasing engine load however at any particular engine load, particulate emissions from mineral diesel were
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Fig. 2. Variations in particulate mass collected on the filter paper with engine load.
relatively higher compared to B20. Biodiesel has inherently superior lubricity [27] compared to baseline mineral diesel therefore it offers lower frictional losses in the engine powertrain. Bulk modulus of compressibility of biodiesel is relatively higher than baseline mineral diesel, which results in marginally advanced fuel injection compared to mineral diesel. In addition, there is fuel oxygen present in biodiesel molecules, which is available for participation in combustion. This improves combustion efficiency [28] and reduces particulate formation. Biodiesel has relatively shorter ignition delay compared to mineral diesel, because it has relatively higher cetane number. Earlier start of combustion of biodiesel droplets results in relatively higher combustion temperature in the cylinder therefore the particulate formed re-burn, before they are emitted in the tail pipe of the engine/vehicle. All these factors lead to lower particulate formation from B20 compared to mineral diesel, under identical engine operating conditions (Fig. 2).
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well as both test fuels and lubricating oil. Particulate sampling was done in triplicate at five engine loads: 0%, 25%, 50%, 75% and 100% at 1800 rpm. SEM analysis was done for the particulate collected on filter paper at rated load using mineral diesel and B20. Trace metal concentrations in diesel, B20 and lubricating oil are shown in Fig. 3. It can be observed from Fig. 3 that Pb, Mg, and Fe were relatively lower in biodiesel compared to baseline mineral diesel. Lubricating oil contained an order of magnitude higher concentrations of Cr, Mg, Ca, and Zn, than mineral diesel and biodiesel. Concentrations of toxic trace metals such as Pb, Cd etc. were lower in biodiesel compared to mineral diesel. Concentration of crustal trace metals such as Ca, Fe, and Mg were higher than that of anthropogenic trace metals such as Cr, Cd, and Pb. Ni was below detectable limits in all samples. Extraction of trace metal elements from particulate samples collected on the filter paper was done using hot plate extraction method. In hot plate extraction method, filter paper strips laden with particulates were placed in a beaker and trace metals were extracted by refluxing on a hot plate, using 10 ml conc. hydrochloric acid (8%) and conc. nitric acid (3%). HPLC grade acid was used, which was free from trace metal contaminants. The extracts were then diluted with mili-Q water to make up 100 ml solutions. The samples were then analyzed in AAS after calibrating the equipment. Concentration of each trace metal was detected three times and an average of three reading was used for calculating the trace metal concentration in the particulates. This data of trace metal concentration in particulates drawn from mineral diesel and B20 fuelled engine at different engine loads is shown in Fig. 4. Each trace metal is discussed separately in the following paragraphs. 3.2.1. Iron Fe concentration was relatively lower in biodiesel particulates compared to mineral diesel particulates. This was mainly because Fe trace concentration was also lower in biodiesel compared to mineral diesel (Fig. 3).
3.2. Trace metals Trace metal elements such as Pb, Cr, Cd, Ni, Mg, Fe, Na, Ca, Zn and Al were analyzed in the particulates collected on the filter papers as
3.2.2. Aluminum Aluminum occurs naturally in soil, water, and air. High levels in the environment can be due to mining and processing of
Fig. 3. Trace metals in mineral diesel, biodiesel and lubricating oil.
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aluminum ores or production of aluminum, alloys, and compounds. Small amount of aluminum is released into the environment from coal-fired power plants and incinerators. Workers who breathe large amounts of aluminum dust can have lung problems, such as coughing or changes that show up in chest X-rays. Al concentration decreased with increased engine load for both fuels. Al traces in particulates were higher compared to Ca, Mg, and Fe. Al traces originate from the wear of aluminum containing engine components such as piston. The trend of Al traces in particulates suggested that B20 fuelled engine underwent relatively lower wear of aluminum containing components compared to mineral diesel fuelled engine and this was because of the inherent lubricity properties of biodiesel. 3.2.3. Lead Lead is one of the most toxic trace metals, whose widespread use affects multiple systems in human body, including its neurological, hematological, gastrointestinal, cardiovascular, and renal development. People get exposed to lead by inhalation of lead containing compounds, which are generated during smelting, unorganized recycling, use of leaded gasoline and leaded paint, leadcontaminated dust typically originating from bearings, water (from leaded pipes), and food (from lead-glazed or lead-soldered containers) [29]. Children are particularly vulnerable to severe neurotoxic effects of lead, and even relatively low levels of exposure can cause serious health effects and in some cases irreversible neurological damage among children. In developing countries, efforts are being made to eliminate non-essential usage of lead such as use of lead in gasoline and paint, ensuring safe recycling of leadcontaining waste, educating public about importance of safe disposal of lead-acid batteries and computers, and monitoring of blood lead levels in children, women of child-bearing age and workers. In these experiments, Pb concentration in the diesel exhaust particulate did not show any trend for these test fuels. Trace concentration of lead in biodiesel particulates was relatively higher at low loads however it was relatively higher in mineral diesel particulates at higher loads. Lead concentration in biodiesel was relatively lower than mineral diesel (Fig. 3).
3.2.4. Magnesium Mg containing dust may irritate mucous membranes or upper respiratory tract. Exposure to magnesium oxide fumes subsequent to burning, welding or molten metal work can result in metal fume fever, which has temporary symptoms such as fever, chills, nausea, vomiting & muscle pain [30]. Mg concentration in B20 particulates at no load was 12.22 mg/g whereas it was 9.50 mg/g in mineral diesel particulates. B20 particulates showed decreasing trend with increasing load because of additional lubricity offered by biodiesel. However in case of mineral diesel particulates, Mg concentration remained almost constant with load. Mg concentration in B20 particulates was lower than mineral diesel at most operating conditions, because biodiesel had lower concentration of Mg traces compared to mineral diesel (Fig. 3). 3.2.5. Chromium Chromium is one of the most widely used metals in industry, such as in steel production, alloy preparation, wood preservation, leather tanning, paints, pigments, metal plating, tanning, electroplating, steel manufacture and other industrial applications [31]. Cr trace concentration in B20 particulate was slightly higher compared to mineral diesel particulates for most loads. This was linked to higher Cr trace concentration in B20 than mineral diesel (Fig. 3). At higher loads, concentration of chromium was relatively lower in B20 particulates because diesel had relatively inferior lubricity properties compared to biodiesel therefore the piston rings, which have Cr coating, wear faster in case of diesel. Thus the Cr traces originating from wear debris at full load were observed in diesel particulates. 3.2.6. Sodium In transesterification process, sodium hydroxide is used as a catalyst. Sodium is also a component of sodium chloride (NaCl), therefore is a very important compound found everywhere in the living environment. Contact of sodium with water, including perspiration causes formation of sodium hydroxide fumes, which are highly irritating to the skin, eyes, nose and throat. This may cause sneezing and coughing. Na trace concentration in B20 particulates was slightly higher
Fig. 4. Trace metals concentrations in exhaust particulates from mineral diesel and B20 fuelled engine.
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than mineral diesel particulates at all loads. Slightly higher concentration of Na in B20 particulates may be due to presence of traces of catalyst (NaOH) in biodiesel, which remains from the transesterification process. Na trace concentration in particulate did not vary significantly with engine load. 3.2.7. Zinc Zinc is found in the air, soil, and water and is present in all foods. Most zinc enters the environment from mining, purifying of zinc, lead, and cadmium ores, steel production, coal burning, and burning of wastes. Inhaling large amounts of zinc (as zinc dust or fumes from smelting or welding) can cause a specific short-term disease called metal fume fever, which is generally reversible once exposure to zinc ceases [30]. Zn trace concentration in exhaust particulates decreased with load for both test fuels. Zn was largely present in the lubricating oil as anti-wear additive and some of these additives deplete with time. Zn concentration in diesel particulate was relatively higher at all engine loads compared to B20 and it suggested that mineral diesel depleted the additives in the lubricating oil at a faster rate compared to B20. 3.2.8. Calcium The distribution of calcium is very wide; it is found in almost every terrestrial area of the world. It is contained in the soft tissue, and in the fluids within the plant tissues. The use of more than 2.5 g of calcium per day without a medical necessity can lead to development of kidney stones and sclerosis of kidneys and blood vessels [30]. Ca trace concentration in mineral diesel particulates was relatively higher for most engine operating conditions. Ca trace concentration generally decreased with increasing engine load. Ca trace concentration was higher in biodiesel compared to mineral diesel. It seems that the lubricating oil consumption played a major role in Ca trace concentration in particulates. 3.3. Benzene soluble organic fraction BSOF analysis for particulate samples was carried out using ASTM D 4600-87 (2001). This method is recommended by National Institute of Occupational Safety and Health (NIOSH), USA to evaluate toxic organic compounds in particulate samples. This test method describes sampling and gravimetric determination of BSOF of the particulate matter. The soluble organic fraction (SOF) of diesel particulates originate primarily from unburned fuel, lubricating oil, and is essentially a product of partial combustion/pyrolysis. Majority of SOF is classified as unresolvable complex material. In the high temperature exhaust system, these SOF compounds remain in gas phase, however upon cooling and dilution while they move downstream in the exhaust system, some of these less volatile organic species adsorb on to the surfaces of the particulates, forming SOF portion of the DPM. The USEPA's Mobile Source Air Toxic (MSAT) list includes seven PAHs namely benzo(a)antracene, benzo(b)fluoranthene, benzo(k)fluoranthene, chrysene, benzo(a)pyrene, indeno(1,2,3-cd) pyrene, and 7,12-dimethylbenz(a)anthracene [32]. Many of these SOF compounds in DPM have shown mutagenic activity in a variety of assay systems, e.g. ethylene, benzene, 1,3butadiene, acrolein and several PAHs. Particle bound PAHs and nitro-PAHs have been the focus of several mutagenic studies, both in bacteria and in mammalian cell systems. PAHs (> 4 rings, e.g., pyrene, benzo[a]pyrene) forming nitro-PAHs and nitro-PAH lactones in atmospheric reactions are major contributors of carcinogens amongst combustion generated toxins. Many nitro-PAHs (> 3 rings, e.g., nitropyrenes) are potent mutagens and carcinogens and
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some reaction products (hydroxylated-nitro derivatives) are mutagenic in bacteria. Nitro-PAHs with more than two rings stay in particle phase (USEPA, 2002). In mammals, PAHs are activated by the enzyme cytochrome P450 dependent monooxygenase, to form dihydrodiol epoxides. These epoxides are capable of forming adducts with DNA, which lead to induction of tumors. Known cancer causing PAHs are pyrene, benzo[a]anthracene, chrysene, benzo[b] fluoranthene, benzo[a]pyrene, benzo[ghi]perylene, indeno[1,2,3cd]pyrene and dibenzo[a,h]anthracene [33]. BSOF is accepted as an indicator of organic fraction of particulates [34]. Organic fraction in particulates is largely composed of toxic compounds therefore BSOF is accepted as a marker of toxicity of diesel exhaust particulates. Particulate sampling was done at five different engine loads: 0%, 25%, 50%, 75% and 100% at 1800 rpm for BSOF determination using mineral diesel and B20 (Fig. 5). Both test fuels showed the same trend i.e. BSOF decreased with increased engine load. This was primarily due to higher in-cylinder temperatures prevailing at higher engine loads, which destroyed BSOF species formed during incomplete combustion in the fuel-rich zones of the combustion chamber. BSOF was found to be higher for B20 particulates at all operating points, compared to mineral diesel particulates, suggesting possibly higher toxicity of particulates emitted by a biodiesel fuelled engine. At idling, BSOF was ~52% in mineral diesel particulates and ~61% in B20 particulates however with increased engine load, BSOF decreased for both test fuels. This phenomenon can be explained by the fact that fuels (Mineral diesel and B20) and lubricating oil undergo partial combustion due to relatively lower temperatures prevailing in the combustion chamber, particularly at low loads, resulting in formation of higher unburned hydrocarbon species, which formed BSOF. Several researchers [35e41] reported that soot is formed in the fuel-rich regions of diesel spray, which collect hydrocarbons through condensation and adsorption process in later stages. Since biodiesel has lower volatility, proportion of BSOF detected in biodiesel exhaust particulates is also relatively higher. Sharp et al. [38] reported that higher BSOF in biodiesel particulates mostly consisted of unburnt esters from biodiesel. It is also possible that higher molecular weight compounds form smaller liquid droplets and adhere to soot. These small liquid droplets may either agglomerate with other droplets or condense on the soot particles or simply get trapped by the filters. 3.4. Regulated gaseous emissions Measurements were done to detect regulated gaseous emission species such as CO, CO2, THC and NOx (Fig. 6).
Fig. 5. BSOF in the exhaust particulates from mineral diesel and B20 fuelled engine.
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3.4.1. THC Both B20 and mineral diesel followed the same HC emission trend with varying engine load. THC emission decreased with increasing engine load. Lower THC emissions from B20 indicated that oxygenated fuel (biodiesel) emitted lower THC during combustion due to presence of fuel oxygen, which led to more complete combustion [13]. 3.4.2. NOx Both nitric oxide (NO) and nitrogen dioxide (NO2) are grouped together and termed as NOx. NO is the dominant oxide of nitrogen formed in engine combustion chamber and its emission mainly depends on engine operating condition and the type of fuel used. Due to presence of oxygen in biodiesel fuel molecules, superior combustion efficiency was obtained because of higher peak combustion chamber temperature. As a consequence, NO formation in the exhaust was relatively higher in B20 engine compared to mineral diesel fuelled engine. At low loads, NO concentration was slightly lower in biodiesel exhaust than mineral diesel exhaust because biodiesel has higher cetane number and superior autoignition characteristics, which led to shorter ignition delay and lower premixed combustion, resulting in slightly lower NOx formation at low loads [42e44].
Fig. 6. Regulated emissions from mineral diesel and B20 fuelled engine.
3.4.3. CO2 CO2 concentration increased with increasing engine load. Both, B20 and mineral diesel showed same trend throughout the engine operating range. CO2 emission from B20 fuelled engine was slightly lower than mineral diesel fuelled engine. This was primarily due to biodiesel being an oxygenated fuel, which was the main reason for its relatively more efficient combustion, resulting in lower CO2 emissions.
Fig. 7. Morphology of B20 and mineral diesel particulates.
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3.4.4. CO CO concentration varied with varying load. Both, B20 and mineral diesel showed same trend throughout the engine operating range. Initially at no load, CO emission was higher but it decreased with increasing load upto 75% rated load for both fuels. Thereafter CO emission increased significantly at full load. CO concentration from mineral diesel engine was slightly higher compared to B20 at all engine loads. Agarwal et al. [25] also reported similar trend for Karanja oil blended with mineral diesel. Inherent oxygen present in biodiesel helped in obtaining relatively more complete combustion, resulting in lower CO emission from B20 at all operating conditions [28]. 3.5. Scanning electron microscopy Morphology of particulates from B20 and diesel fuelled engines at three different engine loads (0%, 50% and 100% rated load) is shown in SEM micrographs (Fig. 7). SEM images were taken at 2000 magnification for all particulate samples collected on the filter papers using a partial flow dilution tunnel. SEM analysis showed that particulate surface morphology changed with engine load. Layered and plate like structures were observed at lower loads, whereas bigger size particles appeared at higher load conditions. From Fig. 7, it is clear that number concentration of particulates seems higher for mineral diesel, while size of particulates seems larger for biodiesel. The sizes of particulates obtained using SEM were sub-micron for biodiesel as well as mineral diesel. 3.6. Energy dispersion X-ray The filter paper containing particulates emitted by mineral diesel and B20 fuelled engine was subjected to energy dispersion Xray (EDAX) analysis in order to detect the elemental composition of particulates collected on the filter paper.. A comparative analysis of trace metals of particulates from diesel and B20 engines using EDAX is shown in Fig. 8. At no load, biodiesel particulates contained higher elemental concentration of trace metals except Pb. However at intermediate loads, Ni, Cr and Ca trace concentrations were lower in biodiesel particulates. At full load, trace concentrations of Pb, Cu, Ni, and Cr were relatively lower in B20 particulates compared to mineral diesel particulates. Cr concentration using both AAS and EDAX methods showed similar trend. In both methods, Cr concentration was found to be -vis relatively lower at high engine loads in case of biodiesel, vis-a baseline mineral diesel. Al and Ca traces concentrations decreased with increasing engine load using AAS method however opposite trend was seen with EDX technique. There were some disagreements in the trace metals in the particulates using AAS and EDAX. However AAS technique was more precise and superior to EDAX. 4. Conclusions Experimental investigations were performed to determine trace metal concentrations and benzene soluble organic fraction (BSOF) of particulates in the exhaust of a medium-duty SUV diesel engine, fuelled by mineral diesel and 20% (v/v) blend of Jatropha biodiesel (B20). Main conclusions from this experimental study are as follows: Biodiesel derived from Jatropha curcus oil showed superior engine performance and emitted lower particulate mass at all engine operating conditions, compared to baseline mineral diesel.
Fig. 8. Trace metal analysis of mineral diesel and B20 particulates using EDAX.
Concentrations of various trace metals in particulates emitted by mineral diesel and B20 were determined using atomic absorption spectroscopy (AAS). Trace metal concentrations in the particulates were lower for B20 compared to baseline mineral diesel at high loads, possibly because of relatively higher lubricating oil consumed by mineral diesel compared to B20. Crust metals (Ca, Fe, Mg and Al) were generally in higher trace concentrations in particulates emitted by mineral diesel engine. However anthropogenic metals (Pb, Cr, Zn, Na) were generally in higher trace concentrations in B20 at lower engine loads and in mineral diesel at higher engine loads. Benzene soluble organic fraction (BSOF), which is a marker of particulate toxicity, was higher in biodiesel particulates at all loads, indicating possibly higher toxicity of biodiesel particulates. Gaseous emissions (CO, HC and CO2, except NOx) were lower from B20 engine compared to mineral diesel engine. NOx emissions were relatively higher from B20 because biodiesel has fuel oxygen. Morphology of exhaust particulates was analyzed by SEM. Particulate size was found to be larger for particulates emitted by biodiesel fuelled engine. Overall, biodiesel particulates showed higher toxicity than mineral diesel however their trace metal concentrations were comparable to mineral diesel trace metal concentrations. GHG
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