Particulate emissions from biodiesel fuelled CI engines

Energy Conversion and Management 94 (2015) 311–330

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Review

Particulate emissions from biodiesel fuelled CI engines Avinash Kumar Agarwal a,⇑, Tarun Gupta b, Pravesh C. Shukla b, Atul Dhar c a

Engine Research Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India Department of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India c School of Engineering, Indian Institute of Technology Mandi, Mandi 175001, India b

a r t i c l e

i n f o

Article history: Received 4 September 2014 Accepted 30 December 2014

Keywords: Particulate Biodiesel Size-number distribution Toxic potential Soot morphology

a b s t r a c t Compression ignition (CI) engines are the most popular prime-movers for transportation sector as well as for stationary applications. Petroleum reserves are rapidly and continuously depleting at an alarming pace and there is an urgent need to find alternative energy resources to control both, the global warming and the air pollution, which is primarily attributed to combustion of fossil fuels. In last couple of decades, biodiesel has emerged as the most important alternative fuel candidate to mineral diesel. Numerous experimental investigations have confirmed that biodiesel results in improved engine performance, lower emissions, particularly lower particulate mass emissions vis-à-vis mineral diesel and is therefore relatively more environment friendly fuel, being renewable in nature. Environmental and health effects of particulates are not simply dependent on the particulate mass emissions but these change depending upon varying physical and chemical characteristics of particulates. Particulate characteristics are dependent on largely unpredictable interactions between engine technology, after-treatment technology, engine operating conditions as well as fuel and lubricating oil properties. This review paper presents an exhaustive summary of literature on the effect of biodiesel and its blends on exhaust particulate’s physical characteristics (such as particulate mass, particle number-size distribution, particle surface area-size distribution, surface morphology) and chemical characteristics (such as elemental and organic carbon content, speciation of polyaromatic hydrocarbons, crustal and anthropogenic trace metals, sulfates, and nitrates) in order to comprehensively assess the effects of biodiesel usage on the environment as well as on the human health. Control of particulate emissions using various engine control parameters such as intake air boosting using turbocharging, high pressure fuel injections, multiple injections, exhaust gas recirculation (EGR), and after-treatment devices in combination with the use of biodiesel has been critically assessed and included in this review article. Ó 2015 Elsevier Ltd. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical characterization of particulates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Chemical composition of particulates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Elemental and organic carbon (EC/OC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Trace metals in particulates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Significance of desulfurized fuel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Unregulated emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1. Carbonyl compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2. Benzene, toluene, ethyl-benzene and xylene (BTEX) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.3. Polycyclic aromatic hydrocarbons (PAHs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.4. Effect of biodiesel on unregulated emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Effect of after-treatment devices (DOC and DPF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. E-mail address: [email protected] (A.K. Agarwal). http://dx.doi.org/10.1016/j.enconman.2014.12.094 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.

312 313 313 314 315 316 317 317 317 317 319 320

312

3.

4.

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

Physical characterization of particulates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Particulate mass emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Effect of injection strategies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Effect of fuel composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Particulate size-number distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Effect of injection strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Effect of fuel composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Fossil fuels have dominated transportation sector since the invention of internal combustion (IC) engines in early nineteenth century. Conventional petroleum resources are finite and they contribute enormously to the ever-rising greenhouse gas emissions in the atmosphere thus renewable alternative fuels are being globally developed and explored continuously by researchers. Depletion of fossil fuels is eminent in the near future. In addition, environmental pollution concerns due to combustion of fossil fuels provide a unique and significant motivation for developing renewable alternative fuels, which have the potential to sustain ever-growing fuel demand for transportation sector. In order to effectively control environmental pollution and mitigate its harmful effects, exhaust characterization at the engine outlet as well as its transformational products in the atmosphere is essential [1]. Health effects of exhaust particulates depend on their chemical composition and physical characteristics, which determine their true residence time and availability as sorption sites inside the human respiratory system [2–6]. Biodiesel has emerged as a strong diesel alternative, which comprises of fatty acid alkyl esters derived from transesterification of triglycerides present in vegetable oils/animal fats. Biodiesel has been well accepted as renewable alternative to mineral diesel globally. A large number of scientific studies have reported successful operation of CI engines with biodiesels derived from different feedstock. Biodiesel can either be used as a full replacement of mineral diesel or it can also be blended with mineral diesel in any proportion [7]. Biodiesel can be produced from various feedstocks such as Soybean oil, Rapeseed oil, Palm oil, Jatropha, and Karanja. Basic composition of biodiesel does not change significantly with change in the feedstock. However, change in fatty acid methyl ester profile leads to change in the physical properties of biodiesel to some extent. Therefore, a slight change in the biodiesel properties can be observed depending on biodiesel feedstocks. Therefore, biodiesel properties are dependent on the feedstock. There have been several studies carried out in the past, which investigated performance, combustion and emissions characteristics of biodiesel fuelled diesel engines [8–14]. Formation of particulates and gaseous emissions depend not only on the physical and chemical properties of the fuels but this process is also greatly influenced by complex in-cylinder processes such as air–fuel mixing, combustion chamber geometry, temperature and pressure conditions of cylinder charge during combustion [15,16]. Particulate formation in a diesel engine is very sensitive to relative air–fuel mixture strength (k) in rich, premixed reaction zones of the combustion chamber, where soot precursors are initially generated [15]. Physical properties of biodiesel are also important, which directly affect spray atomization, droplet size distribution and fuel–air mixing in the combustion chamber. Injection delay and spray tip penetration is relatively longer for biodiesel compared to mineral diesel, whereas spray cone angle, spray area and spray volume are relatively smaller. Relatively higher viscosity and surface tension of biodiesel is responsible for larger

321 321 322 323 324 325 326 327 327

Sauter mean diameter (SMD) of the spray droplets [17]. In the last two decades, advancements in diesel engine technology such as application of very high fuel injection pressures, split injection, turbo-charging, and after-treatment devices have resulted in considerable reduction in engine-out emissions but these technologies have also increased engine control complexities and sensitivity of engine toward changes in fuel properties and lubricating oil properties. It is universally accepted that biodiesel blend usage in CI engines reduces particulate matter (PM) mass emission but mixed trends are reported for the physical and chemical characteristics of biodiesel particulates depending upon the engine technology, biodiesel feedstock, biodiesel blend concentration, type of after-treatment device used, engine management system optimization, etc. Several toxicological studies [2,3] reported that the use of biodiesel in engine results in lower toxicity of particulates, especially lower mutagenic potential as compared to mineral diesel. A review paper [18] on the effect of diesel exhaust on human health concluded that evidence from scientific studies so far is insufficient to adequately validate the diesel particulate-lung cancer hypothesis. This further emphasizes the need to study biodiesel vs. diesel exhaust critically under various engine operating conditions typically encountered in the real world situation. Measurement and toxicity characterization of particulate emissions from automotive engines is still in its development stage. This aspect should be simultaneously looked into along with implementation of new alternative fuels. For emphasizing this aspect, technological developments related to particulate characterization from biodiesel and biodiesel blends have been extensively reviewed in this article. This article is an attempt to present an exhaustive literature review of the effects of biodiesel and biodiesel blends on the exhaust particulate’s chemical characteristics (such as elemental and organic carbon, speciation of polyaromatic hydrocarbons, crustal and anthropogenic trace metals, sulfates and nitrates) and physical characteristics (such as particulate mass emission, particle number-size distribution, particle surface area-size distribution, and particulate morphology). In India, Soybean, Rapeseed, Palm, Jatropha and Karanja based biodiesels are not available commercially as of now. However, some biodiesels are being used on an experimental basis before large scale implementation. Indian railways operates a few of its diesel locomotives on biodiesel blends. This review highlights the possible advantages of using biodiesel commercially on engine exhaust emissions. This study also shows that biodiesel results in lower carbon emissions, lower carcinogenic species emissions, and lower overall carbon dioxide (CO2) emissions and prepares a good ground for its large scale implementation. The review explores the chemical and physical characteristics of particulate emissions from biodiesel fuelled engine, which would certainly be helpful in large-scale application of biodiesel in transport sector in India in future. The world has huge potential for biodiesel production from its waste lands. This review critically assesses the effect of biodiesel on the environment as well as human health under varying engine operating conditions, varying fuel injection parameters and strategies, using

313

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

Fig. 1. Evaporation of single diesel and biodiesel droplet inside combustion chamber [13].

various after-treatment control technologies to assess the possible threats emanating from a large scale implementation of biodiesel as a transport fuel. 2. Chemical characterization of particulates

Ash and Others 14%

Sulphate and Water 13%

Carbon 41%

2.1. Chemical composition of particulates Diesel engine undergoes heterogeneous combustion. Fuel is injected into the combustion chamber toward the end of the compression stroke in conventional CI engines. Modern diesel engines are equipped with common rail direct injection (CRDI) system, which has the capability of split injection, i.e. fuel can be injected in pilot injection, main injection and post injection in the same engine cycle. When fuel is injected into the combustion chamber at very high injection pressure, it breaks into large number of small droplets under the influence of high combustion chamber pressure, prevailing at the end of compression stroke. Compressed air in the combustion chamber offers resistance to the high pressure fuel droplets, which lead to further fragmentation of small droplets into finer droplets. When a relatively larger droplet breaks into several smaller droplets, total surface area of the droplet increases significantly. This higher surface area provides superior interaction between fuel droplets and surrounding hot, high pressure combustion chamber air, which eventually results in higher degree of combustion of the fuel injected. Heywood [19] explained soot formation in the engine combustion chamber. By considering a single fuel droplet inside the combustion chamber, soot formation process was explained. A single fuel droplet comes into contact with hot, high pressure air in the combustion chamber (Fig. 1). A small quantity of fuel evaporates from the droplet surface and forms rich fuel–air mixture closer to the surface. Mixture composition becomes progressively leaner as its distance from the surface of the droplet increases as seen in Fig. 1. Wherever the air availability is good enough for combustion of fuel droplets, combustion tends to be complete, resulting in low particulate formation. Near the droplet surface, where rich mixture forms (k < 1.0), combustion tends to be relatively incomplete, which results in higher particulate formation. Similarly, there is no air/oxygen available inside the core of the droplet. Under the influence of hot burning droplets, fuel closer to the droplet surface and the fuel present inside the droplet is either not able to burn or burn partially, thus forming pyrolyzed fuel remnants, which act as precursors for soot formation. Biodiesel has slightly higher temperature required for vaporization therefore it can be assumed that the vaporization of biodiesel droplet is slightly lower compared to mineral diesel

Unburned fuel 7%

Unburned Oil 25%

Fig. 2. Typical diesel particulate composition [22].

droplet. On the other hand, there is absolutely no oxygen present inside the diesel droplet (Fig. 1a). Absence of oxygen molecules inside the fuel droplet results in unburned hydrocarbons and pyrolyzed carbonaceous compounds. In case of biodiesel, combustion is aided partially inside the droplet (Fig. 1b) by the oxygen atoms present in the biodiesel molecules (11% w/w) [20]. Diesel particulate mainly comprise of four components, namely (i) elemental carbon (EC), (ii) organic carbon (OC), (iii) sulfate and (iv) ash, which mainly includes trace metals [21]. Fig. 2 shows typical composition of particulate emitted by CI engines. EC is known as ‘soot’ and it mainly comprises of ‘carbon’. It is crystalline in structure and mostly forms central part of particulate [23]. OC mainly consists of hydrocarbons, which either remains unburned during combustion, primarily originating from fuel or lubricating oil or form due to condensation of organic vapors left-over from incomplete combustion. Organic fraction of the particulate is of great concern due to its harmful effects on humans. Both diesel and biodiesel exhaust particulates consist of significant amount of organic fraction. However, scientific studies have shown that biodiesel exhaust particulates have significantly lower organic fraction. In a study, Rounce et al. [24] reported that concentrations of acetaldehyde, formaldehyde, benzene, and 1,3-butadiene were lower for Rapeseed methyl ester (RME) in comparison to ultralow sulfur diesel (ULSD). RME produced relatively lower solid particulate and higher liquid particulate as compared to ULSD. Particulate number concentration reduced for the entire size range. However, RME produced a higher proportion of nano-particulates of smaller size range and such nano-particulates had higher

314

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

Fig. 3. Exhaust PM classified by thermo-gravimetric analysis (TGA) [24].

soluble organic fraction (SOF), which is a marker of toxicity. They reported that Diesel particulate filters (DPF) captured 99% solid particulates in terms of mass and particle number for both ULSD and RME (Fig. 3). Similarly, DPF reduced 88% and 80% liquid portion of particulates for ULSD and RME. In another study, Zhu et al. [25] reported significantly reduced smoke opacity with increase in biodiesel proportion in the test fuel, while the total particle number concentration actually increased. It was observed that sulfate and SOF increased in particulates with increasing biodiesel blend concentration, whereas solid particulates actually reduced in number. Nucleation mode particle number concentration was observed to be higher for biodiesel and total particle number concentration reduced with ULSD. The contribution of lubricating oil was suggested to be as high as 80–90% in the SOF portion of the particulate [26]. Chuepeng et al. [27] carried out experiments with B30 and reported that B30 produced lower particulates mass at all engine operating conditions compared to ULSD. They also observed lower EC content in particulate of B30 as compared to ULSD. They suggested that presence of oxygen in biodiesel limits the in-cylinder particle formation by influencing both the carbon chain formation and its oxidation. At a high engine load, higher amount of fuelborne oxygen leads to lower EC formation for biodiesel. Young et al. [28] reported that non-volatile particulate concentration emitted by heavy-duty diesel engine increased with increasing load from 25% to 75% and it decreased with increasing biodiesel blend from 2% to 20%. Williams et al. [85] measured particulate mass for different engine operating conditions and determined volatile mass fraction of the particulates using thermo-gravimetric analysis (TGA) from a CRDI V6 engine fuelled with RME blended with ULSD (B30). They reported higher volatile organic fractions for idle and low load conditions. EC fraction was lower for B30 compared to ULSD. Li et al. [29] compared emissions and particulate size distribution for diesel, fresh cooking oil (FCO) and waste cooking oil (WCO) at two engine operating conditions (23 kW and 47 kW). PM emission was almost equal to diesel at lower load however it reduced significantly when engine was operated at higher loads. FCO showed higher PM emissions at both conditions and both fuels showed lower nuclei mode particles after diesel oxidation catalyst (DOC) compared to mineral diesel. This suggested that DOC was not very effective in reducing nano-particles emanating from diesel compared to those from biodiesel. Schönborn et al. [30] studied the effect of different molecular structures of fatty esters on oxides of nitrogen (NOx) and soot formation. They concluded that different biodiesels have different physical and chemical properties depending on the fatty ester composition, which affect the combustion in diesel engines. Lapuerta et al. [31] evaluated the effect of unsaturation level of biodiesel on NOx and PM emissions. They reported significant reduction in PM mass and smoke opacity for biodiesel (B100) however PM mass

decreased by 20% and NOx increased by 10%, as biodiesel became more unsaturated. They also reported smaller mean diameter particulates from unsaturated biodiesel. Zhang et al. [32] investigated the particle size distribution in a biodiesel blend fuelled CRDI engine exhaust and found that B100 did not emit nucleation mode particles. Soewono and Rogak [33] investigated particulate morphology and microstructure for diesel and B20 by employing transmission electron microscopy (TEM) and Raman spectroscopy. B20 particulates showed higher structural disorder compared to diesel particulates for the same engine operating conditions and structural order of soot improved at higher engine loads. Tan et al. [34] tested five test fuels namely mineral diesel, B10, B20, B50 and B100 for particle size-number distribution and reported that the nucleation mode particles increased with increasing biodiesel blend concentration vis-a-vis mineral diesel. Number of accumulation mode particles decreased with increasing biodiesel content in the test fuel. They explained that higher number of nucleation mode particles emitted by biodiesel may be a result of higher degree of saturation of condensed matter in presence of lesser number of soot nuclei. Evaporation and mixing characteristics of biodiesel is worse than mineral diesel, which leads to an increase in SOF, which forms nucleation mode particles. Biodiesel’s fuel oxygen helps in producing higher number of ultra-fine and nano-particles. Tinsdale et al. [35] investigated the impact of biodiesel on particle numbers, sizes and mass emissions from a diesel engine. Accumulation mode particles and carbonaceous mass decreased and organic mass in particulate increased by using B30. Fatty acid methyl ester (FAME) (B30) led to increased nucleation mode particles as compared to mineral diesel. Song et al. [36] studied particulate emissions from oxidized and heated biodiesel and compared the results with non-oxidized biodiesel and ULSD. They reported that particle mass and particle number reduced with the use of biodiesel for heated and oxidized biodiesel. 2.2. Elemental and organic carbon (EC/OC) Elemental carbon and organic carbon are the two main components of the exhaust particulates. A large number of studies have investigated the EC and OC content of diesel and biodiesel particulates. As a result of heterogeneous combustion in CI engines, unburned and partially burnt hydrocarbons are emitted. Under high temperatures and pressures encountered in the combustion chamber, a fraction of hydrocarbons, which are present in locally fuel rich regions, undergo pyrolysis. Most of the hydrogen atoms get tripped off the hydrocarbon chains and only carbon atoms remain. This results in formation of carbon core, which is also called as ‘soot’ (also known as EC). Carbon undergoes cyclization, and sheet like structure formation and eventually nano-tube like structures called spherules are formed [21]. Volatile organic materials usually condense over the solid and dry soot and the particles grow. This condensed organic material is extremely harmful for humans. This condensed organic matter contains hundreds of organic compounds formed as a result of complex organic species formation pathways during fuel pyrolysis in the engine combustion chamber. Some of the organic compounds are known carcinogens such as polyaromatic hydrocarbons (PAHs), Benzene– Toluene–Ethyl-benzene-Xylene (BTEX). A detailed discussion on PAHs emissions has been included in later part of this review. Poitras et al. [37] observed a significant reduction in PM as well as OC/ EC ratio using biodiesel blends during an experimental study conducted to study the relative impact of B0 (diesel), B2, B5, B10, B20, B50 and B100 on particulate emissions (Fig. 4). Gangwar et al. [38] performed comparative study of diesel and biodiesel PM mass and chemical composition. They reported that OC/EC ratio decreased with increasing engine load. BSOF was higher for B20 compared to mineral diesel for same engine

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

315

Fig. 4. OC/EC emissions, CME biodiesel blends (single marker represents the TPM levels) [37].

operating condition and it decreased with increasing engine load for both test fuels. Although PM emission was lower for B20, PAHs emissions were same for both fuels. Schauer et al. [39] analyzed chemical composition of PM from four gasoline vehicles under three driving cycles namely; cold–cold unified driving cycle (UDC), hot UDC and steady-state driving cycle. They analyzed particulate composition for EC, OC, sulfates, nitrates, organic compounds, etc. using gas chromatography–mass spectroscopy (GC– MS). The average mass emission rates varied from <0.1 to 1.3 mg/km for a hot UDC and steady-state driving cycles, while it ranged between 1.0 and 7.1 mg/km for cold–cold UDC. EC was the main component in particulates for cold–cold UDC cycle and OC consisted of different types of compounds for different cycles. Pabkin et al. [40] also investigated physical, chemical and toxicological characteristics of particulates emitted from a heavy-duty diesel engines equipped with advanced after-treatment devices. They reported significant reduction in particle bound organics from the vehicles equipped with advanced emission control devices. They observed insignificant reduction in hopanes and steranes. Liu et al. [41] used two engine models (2004 model with EGR and 2007 model with EGR) crankcase condenser and a DPF and analyzed particulate samples for C1, C2 and C10–13 particle (EC, OC) phase and semi-volatile organic species. In 2004 model, formaldehyde, acetaldehyde and naphthalene were the major fractions out of 150 analyzed organic species. The concentration of the above compounds reduced significantly in the 2007 model engine. In another study, Agarwal et al. [6] evaluated comparative toxicity of nano-particles emitted from diesel and B20 fuelled engine. OC, EC content of the particulates was determined for primary and secondary emissions from diesel and B20 fuelled engine. A photochemical chamber was used for generating secondary emissions. It was found that EC was higher for diesel for both primary and secondary emissions. EC/OC ratio was also higher for diesel at higher engine loads. Fig. 5 shows the EC/OC ratio obtained for diesel and B20 exhaust [6]. 2.3. Trace metals in particulates Trace metal emissions from CI engines is of a major environmental and health concern. Lubricating oil, fuel and engine friction/wear generated debris are major source of trace metals emitted as a part of particulates from the engines. Concentration of trace metals in the fuels such as mineral diesel varies depending

Fig. 5. EC/OC ratio for diesel and biodiesel (primary and secondary emissions) [6].

upon various factors namely type of crude oil used for production of diesel, synthesis processes and catalysts used in refining process. Trace metals are categorized as ‘anthropogenic metal emissions’ and ‘crustal metal emissions’. Fe, Ca, Mg, Na are the major crustal metal emissions. Several studies have been performed for trace metal emission evaluation from diesel engines. Transition metal containing particulates can even penetrate deep into the human body. These trace metals raise the level of reactive oxygen species (ROS) activity in cell structures, which in-turn elevates the oxidative stress [42–46]. Pillay et al. [47] compared the trace metals in Neem biodiesel vis-a-vis two commercial grade biodiesels and concluded that Neem biodiesel has relatively lower trace metal content compared to the other two. Some metals like Mn, Cu, and Pb were observed to be in higher concentration in Neem biodiesel though. They suggested that further refinement of biodiesel for de-metallization need to be undertaken for sustainable biodiesel usage. Betha and Balasubramanian [48] characterized trace metal emissions from waste cooking oil-derived biodiesel (B100), ULSD

316

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

Fig. 6. Concentration of carcinogenic trace metals in diesel and biodiesel exhaust [48].

Table 1 Trace metals concentration in diesel and biodiesel exhaust particulates [38]. Metals in particulate (mg/g)

No load

Full load

1800 rpm

Na Ca Fe Pb Si Cu Mg B Mn Cr

2400 rpm

1800 rpm

2400 rpm

DE

B20

DE

B20

DE

B20

DE

B20

29.4–29.8 6.22–6.26 12.40–12.44 6.51–6.55 6.75–6.79 2.60–2.64 1.89–1.93 4.30–4.34 0.23–0.27 0.40–0.44

34.6–35.0 11.60–11.64 8.81–8.85 2.31–2.35 2.04–2.08 1.81–1.85 0.81–0.85 3.31–3.35 0.56–0.6 0.05–0.09

51.55–51.59 24.44–24.48 9.62–9.66 2.57–2.61 2.40–2.44 2.13–2.17 1.74–1.78 0.32–0.36 0.057–0.061 0.15–0.19

20.25–20.29 31.70–21.74 8.27–8.31 5.11–5.15 0.63–0.67 2.07–2.11 2.23–2.27 0.07–0.11 0.27–0.31 0.13–0.17

5.50–5.54 1.80–1.84 0.82–0.86 0.61–0.65 1.01–1.05 1.89–1.93 1.52–1.56 0.37–0.41 0.01–0.03 0

11.0–11.4 2.27–2.31 3.26–3.30 1.00–1.04 0.84–0.88 1.93–1.97 0.57–0.61 1.50–1.54 0.17–0.21 0.01–0.03

1.91–1.95 0.39–0.43 0.74–0.78 0.33–0.37 1.14–1.18 0.31–0.35 0.27–0.31 0.14–0.18 0.01–0.05 0.01–0.03

1.37–1.41 0.22–0.26 0.32–0.36 0.61–0.65 0.04–0.08 0.11–0.13 0.04–0.08 0.01–0.03 0.01–0.03 0.01–0.03

and B50. Mg, K, and Al were present in significantly higher concentration in diesel as well as biodiesel. In biodiesel, Zn, Cr, Cu, Fe, Ni, Mg, Ba and K were observed to be in higher concentration compared to biodiesel. However, Co, Pb, Mn, Cd, Sr, and As were observed to be higher in mineral diesel. They carried out risk assessment study and found that B100 exhaust has possibly higher health risk compared to ULSD. Fig. 6 shows the emission of anthropogenic trace metal emissions from ULSD, B50 and B100 [48]. Gangwar et al. [38] evaluated trace metal content in diesel and Karanja biodiesel (B20) particulates. They observed that Si, Cu, and Mg were present in higher concentration in diesel particulate as compared to biodiesel particulate. However, Na, Fe, Ca, Pb, Mn and Cr were found to be higher in biodiesel particulate as compared to diesel particulate. Table 1 shows the metal concentration in diesel and Karanja biodiesel particulates. Agarwal et al. [7] also determined trace metal concentrations in diesel and Karanja oil and reported relatively higher concentration

of Ca, Co, Cr, Cu, Fe, Mg, Ni, Pb and Zn in mineral diesel compared to Karanja oil. 2.4. Significance of desulfurized fuel Diesel is a petroleum product derived from crude oil. Crude oil contains significant amount of sulfur ranging from 0.5% to 5.0% (w/ w) [49]. For automotive applications, sulfur content of diesel should be very low therefore diesel’s sulfur content is reduced significantly during refining process. Several studies showed that sulfur content of diesel increases formation of particulate matter. Sulfur leads to formation of sulfur dioxide, which results in formation of sulfates. Sulfates acts as nuclei for the condensation of volatile organic compounds present in the diesel exhaust. The process, in which condensed matter get adsorbed onto the existing nuclei is called heterogeneous nucleation. On the other hand, main advantage of biodiesel is that it has no sulfur [20] hence sulfate origin particulates are formed. Modern diesel vehicles equipped with

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

after-treatment devices like DOC, and DPF are vulnerable to sulfur content of the fuel. Catalysts like platinum (Pt) promote formation of sulfate particulates in diesel exhaust, which is undesirable. 2.5. Unregulated emissions The current emission legislations control emission of regulated gases (CO, THC and NOx) and particulate matter (PM). Vehicles/ engines also emit large number of other emissions, most of which are in very small quantities except CO2 and moisture, which are categorized as unregulated emissions. In some emission legislations, CO2 is now included as regulated species. Unregulated emissions are important from health stand point. Ravindra et al. [50] indicated in their research that there should be emission regulations for carcinogenic compounds like PAHs. PAHs, carbonyl compounds and BTEX are harmful species emitted by diesel engines in traces. It is important to perform chemical speciation of these organic species being emitted by the engine. 2.5.1. Carbonyl compounds Diesel engines emit large number of different harmful compounds and many compounds are still unknown. The term carbonyl refers to the carbonyl functional group, which is a divalent group consisting of a carbon atom double-bonded to oxygen. Carbonyls are such compounds, which have significant presence in engine exhaust. Most studies have measured carbonyl emissions by derivatives of 2,4-di-nitro-phenyl-hydrazine (DNPH) [51–55]. Carbonyl emissions lead to formation of secondary organic aerosols (SOA) by forming oligomers [56]. Contribution of carbonyls in diesel particles also enhances their response physiologically [57]. Fig. 7 shows the basic structures of carbonyl groups and carbonyl compounds such as aldehydes and ketones. Pang et al. [58] investigated characteristics of carbonyl emissions from a diesel engine fuelled with biodiesel–ethanol–diesel blend. They reported that acetaldehyde was the carbonyl compound emitted in highest concentration, followed by formaldehyde, acetone, propionaldehyde and benzaldehyde respectively. They reported 1–12% higher total carbonyl emissions with biodiesel–ethanol–diesel blend depending on engine operating condition. They also observed that carbonyl emissions increased with increasing engine speed while minimum carbonyl emissions were found at 50% engine load, when the engine was operated at a constant speed [59]. Ho et al. [59] measured and quantified 15 different carbonyl species and formaldehyde was the most dominant compound, followed by acetaldehyde and acetone. They reported that formaldehyde was 54.8–60.8% of the total carbonyl compounds present in the exhaust. They took samples at various locations in the city of Hong Kong and reported that formaldehyde concentration was quite high compared to theoretical value expected in summer, which suggests significant effect of photochemical reactions in formaldehyde production in the ambient atmosphere.

Fig. 7. Carbonyl group, aldehyde and ketone.

317

2.5.2. Benzene, toluene, ethyl-benzene and xylene (BTEX) Petroleum product such as gasoline contains these compounds (BTEX), which have harmful effects on humans. Cheung et al. [60] investigated BTEX emissions from a diesel engine fuelled with mineral diesel, biodiesel and biodiesel blends with methanol (5%, 10%, and 15%) at constant engine speed of 1800 rpm at five different loads. They reported that biodiesel had lower BTEX emissions compared to mineral diesel and higher blends of methanol in biodiesel further reduced BTEX emissions. Higher oxygen content in the fuel leads to oxidation of BTEX. They observed that higher engine load resulted in lower BTEX emission in the engine exhaust. Di et al. and Takada et al. [61,62] also reported lower BTEX emissions at higher engine loads. Ballesteros et al. [63] used biodiesel and reported relatively lower aromatic emissions. Machado Corrêa and Arbilla [64] found a strong correlation between carbonyl emissions and biodiesel content (r2 > 0.96). They reported that esters in biodiesel may be a main source of these carbonyl emissions. On the other hand, Liu et al. and Cheung et al. [41,60] indicated that carbonyl emissions increase with increasing biodiesel content at lower engine load, however they decreased at higher engine loads. Xue et al. [65] summarized that biodiesel reduces the emission of aromatic and poly-aromatic compounds. They also suggested that carbonyl emissions increase in general with biodiesel content because biodiesel provides extra inherent oxygen in fuel molecules. 2.5.3. Polycyclic aromatic hydrocarbons (PAHs) PAHs are well known carcinogens and are produced as a result of incomplete combustion of fuel in diesel engines. Ravindra et al. [50] prepared a database to identify and characterize the PAH emissions in their study. They also discussed factors affecting PAH emissions. Most of the probable human carcinogenic PAHs were found adsorbed on to the PM surface. There are no strict regulations for PAH emissions but these pollutants should get high priority due to their negative impact on human health. Fig. 8 shows the priority listed PAHs. Some of compounds shown in Fig. 8 are considered as ‘probable human carcinogen’ (B2), while some are not listed as ‘human carcinogens’ (D) [50]. The toxicity of these PAH compounds is highly dependent on their molecular structure. Two isomers of PAHs with different structures show quite different toxicity. Therefore EPA has divided these PAH compounds into different categories. Lea-Langton et al. [66] collected particulate samples for diesel, biodiesel and cooking oil for comparison and analysis of particulate bound PAH emissions from a heavy duty DI diesel engine. Most of the particulate bound PAHs were found to be lower in both biofuels compared to mineral diesel, especially at low load conditions and most of the larger PAHs such as benzo(a)anthracene, chrysene, benzo(b)fluoranthene and benzo(k)fluoranthene were oxidized by DOC. They also reported that fluoranthene was absent in mineral diesel but was present in particulates, which was an evidence of pyrolytic formation of fluoranthene in engine combustion chamber. Zielinska [1] reviewed physical and chemical transformations of primary diesel emissions. They concluded that transformation of primary diesel emissions in atmosphere is very important in the context of human health. Primary diesel exhaust reacts mainly with OH radicals, ozone, NOx radicals and sunlight. Monocyclic aromatics of primary diesel exhaust reacts with OH radicals and form various aromatic compounds such as phenols, glyoxal, quinones, nitro-PAHs, and aromatic aldehydes. Agarwal et al. [6] compared the toxic potential of diesel and biodiesel (B20) for primary and secondary emissions. By measuring particulate size-number distribution, size-surface area distribution, EC/OC content, particle bound PAHs, and toxic equivalent factor, toxicity and potential health hazards of these emissions were assessed. They reported that toxicity of biodiesel exhaust was comparatively lower than mineral diesel exhaust. Lu et al. [67] reported that waste cooking biodiesel resulted in reduction of PAH emissions in comparison

318

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

Fig. 8. Priority list for polyaromatic hydrocarbons (PAHs) [50]. ⁄ (not included in priority list); D (not listed as human carcinogens); B2 (probable human carcinogen).

to LSD and ULSD. They reported that ULSD resulted in nearly 8.6% lower PAH emissions compared to LSD. Biodiesel significantly reduced the PAH emissions in the particulate as compared to ULSD and LSD and it was reported to be lower by 32.5% and 38.1%, respectively compared to LSD and ULSD. In recent decades, it is reported that PAHs presents in the diesel particulates are one of the main factor, which adversely affect human health. PAHs include various PAHs, which manifest different toxic properties. EPA has listed 16 PAHs as carcinogenic, probable carcinogenic and possible carcinogenic and the molecular structure of these is shown in Fig. 8 [50]. Researchers [68,69] have performed speciation of PAHs adsorbed on the diesel particulates. Each PAH has a different toxic potential for carcinogenic effects therefore speciation of PAHs is important [70]. There are some studies, which report the toxic potentials of individual PAH species. Agarwal et al. [6] calculated individual PAH content from the total PAH load by using the procedure given by Pan et al. [69] (Fig. 9). They also evaluated the toxic equivalent factors (TEFs) of 8 PAHs and 2 nitro-PAHs. They performed this experimental study on diesel and biodiesel (B20) for primary and secondary emissions from a CRDI engine. For secondary emissions, they used a UV light illuminated photo-chemical chamber with a 2 h residence time. They observed that the trend of total toxic equivalent potential was similar to the particle bound PAH emissions. Slightly higher toxic potential for diesel was observed compared to biodiesel. Similarly, primary particulates showed lower toxicity compared to secondary emissions in terms of PAH toxicity.

Fig. 9. Total toxic equivalent potential of PAHs emitted by diesel and biodiesel (B20) fuelled engine [6].

Karavalakis et al. [71] studied the impact of five biodiesels on PAHs, nitro-PAHs and oxy-PAHs emissions. 10% (v/v) biodiesels were blended with mineral diesel. A Euro-3 CRDI engine was tested on New European Driving Cycle (NEDC) and results were compared with Artersias Driving Cycle (ADC). They observed that biodiesel addition to mineral diesel led to increase in emission of lower

319

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

performed experiment on a Cummins B5.9 engine. PAHs and nitroPAHs emissions for diesel and biodiesel blends were obtained by gas chromatography (GC) coupled with mass spectrometry (MS). Bagley et al. [78] reported that particle-bound PAHs and 1-nitropyrene reduced by use of biodiesel.

Fig. 10. Toxicity equivalent factors (TEFs) for diesel and biodiesel blends [65].

molecular weight PAHs. This indicated relatively lower toxicological potential of biodiesel blends. However, higher molecular weight PAHs showed both increasing and decreasing trends. It was observed that nitro-PAHs were higher for biodiesel blends and oxy-PAHs increased with increasing biodiesel blend concentration. Increasing engine speed and load reduced emission of most PAHs. Karavalakis et al. [68] tested Euro-2 diesel engine with diesel and different biodiesel blends (B5, B10 and B20) for two driving cycles (ADC and NEDC). They reported 11 PAHs and 5 nitro-PAHs emissions in the exhaust from diesel and biodiesel blends. Lower molecular weight PAHs like phenanthrene, anthracene, pyrene were the dominating PAHs, when biodiesel was blended with diesel. In general, biodiesel blending resulted in lower emission of PAHs and nitro-PAHs. Bakeas et al. [72] investigated PAH emissions from a Euro-4 CRDI engine fitted with DOC under NEDC and ADC driving cycles (Fig. 10). They used Soybean biodiesel, a Palm-based biodiesel and an oxidized biodiesel obtained from used frying oils, which were blended with ULSD (B30, B50 and B80). They observed that all biodiesels reduced overall PAH toxicity except waste frying oil, which offsets the advantage of using waste frying oil as feedstock for biodiesel production (Fig. 10). Ravindra et al. [50] reported that larger molecular weight PAHs formed by pyro-synthesis of lower molecular weight PAHs in addition to contributions from lubricating oil. Rhead and Hardy [73] explained that PAHs are complex organic molecules having hydrogen and carbon atoms and at least two benzene rings. PAH formation takes place because of the incomplete fuel combustion, and unburned lubricating oil [50,74]. Riddle et al. [75] showed that PAHs are mutagenic and carcinogenic. Miet et al. [76] explained that nitro-PAHs are formed in the engine as precursor PAHs due to incomplete combustion. Nitro-PAHs can also be formed as a result of radical reactions of OH- and NO-3 with PAHs. Heeb et al. [77] showed that nitro-PAHs contribute to mutagenicity and genotoxicity of diesel particulates. Nisbet and LaGoy [70] reported toxic equivalent factor of different PAH species. Pan et al. [69] compared the PAH and nitro-PAH emissions from diesel, biodiesel (B100, soy methyl ester), and B20. They

2.5.4. Effect of biodiesel on unregulated emissions Karavalakis et al. [68] carried out investigations on regulated and unregulated emissions from a Euro-2 indirect injection (IDI) diesel passenger vehicle (Toyota Corolla 2.0 TD CR: 23:1, 61 kW@4000 rpm 174 N m@2000 rpm, 1998 model) using LSD and soy methyl ester blends and compared the results of experiments performed under ADC and NEDC test cycles. Unregulated emissions of PAHs, nitro-PAHs and carbonyl compounds were measured. For PAH analysis, glass fiber filters were used for particulate phase sampling. Gas chromatograph mass spectrophotometer (GC–MS) was used for PAHs and nitro-PAHs determination. They identified and quantified 13 carbonyl compounds in the exhaust and reported relatively higher concentration of carbonyl compound in ADC compared to NEDC. Formaldehyde was the major compound in both cases, followed by acetaldehyde, butyraldehyde, benzaldehyde, valeraldehyde and p-tolualdehyde. Formaldehyde mainly originated from incomplete combustion of saturated aliphatic hydrocarbons. Lower saturated aromatic hydrocarbons in the biodiesel blends were responsible for lower Formaldehyde emissions from higher biodiesel blends. Thus, carbonyl emissions were affected by biodiesel blending ratio. Tan et al. [79] performed experiment on a light duty diesel engine using five different fuels with different sulfur content and investigated effect of sulfur on regulated and unregulated emissions. The investigations were conducted for three unregulated emissions namely formaldehyde, acetaldehyde and sulfur dioxide (SO2). Formaldehyde was not detected by their instruments. Acetaldehyde emission decreased with increasing load and decreased with increasing fuel sulfur content. SO2 emission increased continuously with increasing engine load and decreased with lowering sulfur content of the fuel. Concentration of formaldehyde was so low that it could be measured only at low engine loads. This suggested that low combustion chamber temperature prevailing at low load condition is the main reason for higher formaldehyde emission. Formaldehyde is an intermediate combustion product. Formaldehyde emission decreased with increasing engine load and combustion chamber temperature [79]. Cheung et al. [80] conducted an experiment on a four cylinder DI engine with ULSD and four different ethanol blends (blend-1, blend-2, blend-3 and blend-4 containing 6.1%, 12.2%, 18.2% and 24.2% ethanol (v/v) i.e. oxygen content 2%, 4%, 6% and 8% (w/w)) (Table 2). Ethanol is an oxygenated compound, and can be blended with diesel. The objective of this study was to investigate regulated and unregulated emissions from ULSD-ethanol blends. They found that unburnt ethanol and acetaldehyde emissions in engine exhaust increased but formaldehyde, ethene, ethyne, 1,3-butadiene and BTX decreased with increasing load (Fig. 11). They observed that formaldehyde emission decreased with increasing

Table 2 Ethene, ethyne and 1,3-butadiene emissions at various engine loads [80]. mg kW h

ULSD Blend-1 Blend-2 Blend-3 Blend-4

1

0.20 MPa

0.38 MPa

0.55 MPa

C6H6

C7H8

C8H10

C6H6

C7H8

C8H10

C6H6

C7H8

C8H10

79.2 95.9 97.2 112.6 153.0

17.1 8.7 9.1 9.4 13.8

69.7 58.3 38.0 55.3 66.8

57.0 54.0 53.0 48.9 63.4

8.3 4.3 4.3 4.1 5.8

33.2 28.9 18.2 24.5 25.3

28.1 23.6 20.8 26.0 30.3

3.3 2.6 2.5 1.9 3.2

18.7 17.0 10.6 12.5 13.4

320

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

Fig. 11. Effect of ethanol and engine load on (a) unburned ethanol, (b) formaldehyde, and (c) acetaldehyde emissions [81].

Table 3 Effect of DOC on fuel based emission factors (EF) of TC, EC and OC in PM2.5, diesel fuel [83]. Speed (rpm)/load%

2125/25 2125/50 2125/75 2690/25 2690/50 2690/75

EC/(mg/g)

OC/(mg/g)

TC/(mg/g)

Diesel

Diesel + DOC

Diesel

Diesel + DOC

Diesel

Diesel + DOC

0.016 0.05 0.084 0.032 0.135 0.206

0.007 0.037 0.082 0.029 0.12 0.134

0.15 0.148 0.144 0.306 0.23 0.221

0.108 0.118 0.073 0.228 0.162 0.182

0.165 0.198 0.229 0.337 0.365 0.427

0.115 0.155 0.155 0.258 0.282 0.317

alcohol content in ULSD, possibly because of increased H/C ratio [81]. However, emissions depend on several factors such as fuel composition, oxygen content, engine technology, and test cycles. They also measured BTX emissions. BTX emissions reduced with increasing engine load. This was because at higher combustion chamber temperature, benzene and its derivatives oxidize. At low engine load, higher benzene emissions were observed. Toluene and xylene showed same trend as benzene. Combustion chamber temperature and oxygen content of fuel are therefore very important factors for BTX emissions. 2.6. Effect of after-treatment devices (DOC and DPF) Biodiesel is an alternate fuel for mineral diesel however it has some properties, which are quite different from diesel. Some studies [6,82] showed that particulate characteristics of biodiesel are

different than mineral diesel. Several researchers performed studies on the effect of biodiesel exhaust on the after-treatment devices. Biodiesel contains trace metals, which lead to catalytic activity. Composition of biodiesel particulates is quite different compared to ones from diesel. Agarwal et al. [6] suggested that biodiesel contain complex compounds, which are relatively more difficult to oxidize during combustion. Shi et al. [83] investigated the effect of DOC on exhaust from engine fuelled with mineral diesel and B20 and reported that DOC reduces CO and HC emissions by 90–95% and 36–70% respectively (Table 3). Total carbon emission decreased by 22–32% with use of DOC. OC reduction was 35– 97% and EC reduction was 3–65%. OC/EC ratio of PM2.5 from mineral diesel slightly increased at lower loads after DOC, however this parameter showed an opposite trend forB20. Zhu et al. [84] evaluated particulate and unregulated emissions with and without DOC in a Euro-5 diesel engine fuelled with biodiesel and biodiesel–ethanol blends. DOC was quite effective in

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

reducing PM mass emissions, total particle numbers as well as unregulated emissions. However, DOC was not equally effective in emission reduction for hydrocarbon compounds. They concluded that combination of biodiesel–ethanol with DOC is effective in reducing particulate emission and unregulated emissions. Bagley et al. [78] tested an IDI diesel engine for emission reduction with oxidation catalytic converter fuelled with diesel and soy biodiesel. They reported that vapor phase PAH emissions reduced up to 90% by the use of oxidation catalytic converter for both fuels. Particle and vapor phase mutagenic compounds reduced up to 50% by use of oxidation catalytic converter. Williams et al. [85] investigated adverse effects of trace metals present in biodiesel on the after-treatment devices, which included several DPF substrates, DOC and selective catalytic reduction (SCR) catalysts. They observed no thermal–mechanical degradation of cordierite, aluminum titanate or silicon carbide DPFs with 150,000 miles equivalent exposure to biodiesel ash and thermal aging. Performance of DOC was adversely affected at 150,000 miles equivalent aging, and resulted in increased level of HC and CO emissions after DOC. Vertin et al. [86] conducted tests to observe the impact of soy biodiesel blend (B20) and two types of ULSD on a cordierite DPF. They observed that B20 particulates were more reactive to DPF compared to diesel particulates. DPF showed 80% higher efficiency with B20 and pressure drop was also not very high. Austin et al. [87] performed active regeneration experiment on a B20 fuelled diesel engine equipped with DOC and DPF. B20 particulates showed five times greater reaction ratio in active regeneration compared to ULSD. The researchers concluded that due to the higher reaction rate of B20 particulates, lower amount of fuel is required for regeneration. Asti et al. [88] studied the effect of biodiesel on particulates during active and passive DPF regeneration and found that particulate emissions decreased with increasing biodiesel content in the fuel. They observed temperature gradient in DPF during active regeneration with biodiesel, however, no appreciable temperature gradient was observed during passive regeneration with biodiesel. Ash content of DPF was also higher with biodiesel. Parihar et al. [89] focused on physical characterization of diesel and biodiesel particulates from a CRDI engine using MOUDI (10 stages) and observed that submicron particle mass concentration was higher for higher loads. PM2.5 contributes approximately 75–90% of the total particulate mass. Di Iorio et al. [90] analyzed the impact of biodiesel on particulate emissions and DPF regeneration and observed that biodiesel leads to lower particulate emissions, which require less frequent regeneration. On the other hand, regeneration of DPF with biodiesel requires higher quantity of fuel to be injected due to biodiesel’s lower calorific value. They suggested that using a ‘flexible management system’ is required for optimum regeneration, which can take care of differences in fuel properties. Pidgeon et al. [91] investigated the effect of biodiesel blend on catalyzed particulate filter (CPF) performance and observed that the soot reactivity of the CPF increases with increasing blend ratio of biodiesel. They also reported that the PM oxidation increases with increasing biodiesel blend ratio at constant CPF temperature. It can safely be stated that use of biodiesel results in overall reduction in PM mass emissions. Carbonaceous particulate suspended in the atmosphere act as black body and absorb heat from the solar radiations. This absorbed heat increases the atmospheric temperature. Therefore, lower PM mass emissions from biodiesel fuelled vehicles offers advantages for the environment. Biodiesel is produced from feed-stocks such as Soybean, Karanja, and Jatropha. These plants consume CO2 from the environment in photosynthesis process while growing, which is released when these fuels are used. On the other hand, fossil fuels emit CO2 during their usage and this increases green-house gas emissions into the atmosphere irreversibly. Studies show that biodiesel production and utilization emits 45–65% lesser CO2 into the environment

321

compared to conventional diesel, which is desirable due to lowering of greenhouse effect. Biodiesel and its blends with mineral diesel offer advantages over mineral diesel, especially in terms of EC/OC emissions, trace metals, sulfur content and unregulated emissions. Most scientific studies showed that biodiesel significantly reduces PAHs and BTEX emissions. Few specific trace metals in biodiesel enhance catalytic effect of particle oxidation and this is important in after-treatment devices such as DPF. Use of biodiesel leads to overall reduction in particulate mass emission in exhaust. Consistent use of biodiesel reduces the particulate load on the after-treatment devices, which increases their effective life significantly. On the basis of chemical characterization of biodiesel particulates, it can safely be concluded that biodiesel reduces PM emissions as well as toxicity. 3. Physical characterization of particulates Biodiesel is a good alternative fuel for mineral diesel. As a partial or complete replacement of mineral diesel, biodiesel needs to be critically evaluated for its physical, chemical and thermal properties as well as its combustion products/emissions. Physical characterization of emissions include measurement of the emitted particulate’s mass and size-number distribution. Sections 3.1 and 3.2 provide insights into particulate mass and number emissions from diesel engine fueled with biodiesel and its blends with mineral diesel. 3.1. Particulate mass emissions Diesel engines are one of the biggest source of carbonaceous particulate emissions in the environment. These primary particulate emissions cause several adverse effects in the environment as well as on human health. These particles are formed due to incomplete combustion of fuel in fuel-rich regions of the diesel engine. Formation of particulate occurs mainly due to the insufficient oxygen availability in fuel-rich regions during heterogeneous combustion. It is therefore essential to optimize the fuel injection parameters in order to reduce particulate formation. Biodiesel is an oxygenated fuel and some of its properties are different than mineral diesel, causing lower particulate formation in an engine. Particulate mass emission is very important from regulated emission control point of view and has been explained in two different aspects. First, the effect of fuel injection strategy, and second, the effect of biodiesel blend composition of particulate emission. Most studies on biodiesel show lower particulate mass emissions compared to mineral diesel. Biodiesel fuel’s molecular oxygen helps in improving combustion in the engine combustion chamber thus lower particulate emissions are reported [31,92] however some studies have also reported identical or higher PM mass emissions with biodiesel [93,94]. An EPA report encompassing 39 scientific studies on heavy duty engines concluded that use of B100 and B20 leads to 50% and 10% PM mass reduction respectively visa-vis mineral diesel [92]. A scientific study suggested that reduction in smoke from Neem oil methyl ester (NOME)-diesel and castor oil methyl ester (COME) – diesel blends was mainly due to additional oxygen present in biodiesel, which reduced PM formation [95]. Biodiesel blended with LSD emitted lower PM mass emission compared to LSD, however no concrete trend was reported for biodiesel blended with ULSD compared to ULSD [96]. Another scientific study reported relatively lower PM mass emissions from 100% biodiesel and biodiesel blends in comparison to mineral diesel in a DI engine equipped with electronically controlled fuel pump [97]. They also suggested that fuel oxygen in biodiesel/ blends was the main reason for reduced PM mass emission [97]. Another study reported that Soy biodiesel usage resulted in 77%

322

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

Table 4 Summary of effect of biodiesel on PM mass emissions.

a

Test fuel

Change in PM (% w/w)

Reference

B35 (Soybean oil biodiesel) B100 (Soybean oil biodiesel) B100 (Soybean oil biodiesel) B100 (Soybean oil biodiesel) B100 (Soybean oil biodiesel) B10 (Rapeseed oil biodiesel) B20 (Rapeseed oil biodiesel) B100 (Rapeseed oil methyl ester) B100 (mixture of Rapeseed and recycled cooking oil methyl ester) B100 (Karanja oil biodiesel) B10 (Palm oil biodiesel) Ethanol:methyl soyate:diesel (5:20:75) blend B100 (yellow grease biodiesel) B50 B20

25 55a 60a 77 52a 24 0 50 80 to 50a 17 30 64a 12a 13a

Wang et al. [100] Canakci [101] Canakci and Van Gerpen [102] Sharp et al. [98] Qi et al. [103] Kousoulidou et al. [104] Turrio-Baldassarri et al. [93] Krahl et al. [105] Grimaldi et al. [106] Raheman and Phadatare [107] Kousoulidou et al. [104] Shi et al. [108] Canakci and Van Gerpen [102] Xiaoming et al. [109] Xiaoming et al. [109]

65a

Change in smoke opacity.

Fig. 12. Mean and 95% confidence interval of PM emissions [100]. Fig. 13. Effect of biodiesel blending on PM emissions [115].

lower PM mass emission compared to mineral diesel [98]. Correlations derived from several test fuels suggested that PM mass emission decreased with increasing fuel oxygen content as well as adiabatic flame temperature [98]. One study on RME reported that particulate emissions significantly depend upon engine operating conditions. At low engine loads, relatively higher PM mass emissions were observed with biodiesel compared to mineral diesel. Interestingly, at higher engine loads, a reduction in PM mass emission was observed with biodiesel [99]. Table 4 summarizes several scientific studies showing the effect of different biodiesels on PM mass emission vis-a-vis baseline mineral diesel. Wang et al. [100] performed tests with diesel and 35% biodiesel blend and reported approximately 25% reduction in PM emissions for B35 (Fig. 12). Another study reported that both, B100 and B20 reduced PM emissions, and PM reduction is generally independent of the feed stock used for biodiesel production. Even lower blends of biodiesel were effective in reducing PM mass emissions [110]. Lower particulate emissions were also seen for biodiesel fuelled engine fitted with an oxidation catalyst [111]. Reduction in smoke was also reported with increasing blend concentration of Linseed oil biodiesel in another study [112] and higher reduction in smoke level was reported at higher engine load [113]. Lower PM mass emissions were also reported for 100% Palm oil biodiesel vis-a-vis mineral diesel [114]. A review article on criteria pollutants from biodiesel fuelled engines concluded that increased biodiesel concentration in test fuel in a heavy-duty diesel engine application reduces HC, CO, and PM emissions substantially, along with slightly increased NOx emissions [115]. Fig. 13 shows these findings for heavy-duty

and medium-duty engines. Results from light-duty engines were more fluctuating, and showed some increase in CO, PM and NOx emissions with increasing biodiesel concentration in the test fuel. Another scientific study reported that PM mass emissions mostly decreased with increasing biodiesel content in the test fuel at all test conditions, reaching peak reduction of 49–62% with 100% biodiesel vis-a-vis baseline diesel [37]. Saanum et al. [116] reported that PM emissions were slightly higher for biodiesel than diesel (marine gas oil: MGO) at low engine loads, but the trend reversed at higher engine loads and biodiesel emitted lower PM mass emissions compared to mineral diesel. The filter smoke number (FSN) was significantly lower for biodiesel at all loads tested.

3.1.1. Effect of injection strategies It is desirable to have superior combustion in diesel engines to have lower particulate emissions. This can be achieved by optimizing injection parameters such as fuel injection pressure, and injection timing. A diesel engine with mechanical pump system does not provide flexibility in injection therefore modern diesel engines are equipped with CRDI system, which provides excellent flexibility for fuel injection such as high pressure (up to 2000 bar) injection, split and multiple injections, injection duration control and injection timing control. PM mass emissions can be drastically reduced by optimizing fuel injection parameters [117]. Many studies have reported significant reduction in PM mass emissions upon using modern high pressure flexible fuel injection systems. This section provides insights into the effect of different injection

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

strategies on particulate emission from CI engines fuelled with mineral diesel and biodiesel. Suryawanshi and Deshpande [118] reported that for various blends of Karanja oil methyl ester, smoke opacity was lower compared to mineral diesel operation. Smoke was further reduced by retarded injection timings. Corgard and Reitz [119] observed that utilization of biodiesel blends resulted in lower smoke emission from a high-speed direct injection (HSDI) diesel engine. They retarded the injection timings for reducing high NOx levels due to biodiesel. Nearly two-third reduction in smoke produced by mineral diesel was achieved by using 30% biodiesel blend with retarded injection timings [119]. Combustion chamber visualization and computational studies on soot formation confirmed that it gets accumulated in the spray tip region [120–123]. In single injection pulse, high momentum of injected spray droplets for longer duration continuously results in supply of fuel droplets to relatively low temperature regions of the combustion chamber in comparison to split injection case, which results in higher soot formation [120]. In a split-injection mode, the fuel injected in the second pulse, wherein fuel droplets enter into a relatively fuel-lean and high-temperature region formed due to the combustion of fuel injected during the first injection pulse. In split injection, soot formation is considerably lower

Fig. 14. Temporal change of non-evaporated spray for gas oil (JIS-2D), recycled corn oil (BIO-1) and recycled Rapeseed oil (BIO-2) [124].

323

because the fuel injected is rapidly consumed by the combustion process before it starts accumulating in a fuel-rich soot-producing zone [120]. For achieving effective reduction in soot formation, split injection, and optimization of time interval between the two injection pulses are very critical. Separation between two injection pulses should be long enough such that the soot formation zone of the first injection pulse is not replenished with the incoming fuel from the second injection pulse. Separation between the two injection pulses should be short enough to ensure that high temperature and pressure conditions are available to the incoming fuel droplets for their rapid combustion, resulting in lower soot formation [120]. Yamane et al. [124] observed that PM emissions from biodiesel showed a high level of SOF compared to mineral diesel (gas oil) at lower engine loads. They investigated spray jet penetration of biodiesel and mineral diesel corresponding to injection conditions prevailing at lower engine loads. They observed shorter spray penetration of biodiesel droplets due to its higher kinematic viscosity and density, which results in inferior air–fuel mixing [124] (Fig. 14). Inferior air–fuel mixing was reported to be the main reason for higher PM mass emissions. Ye and Boehman [125] concluded that impact of injection strategy and biodiesel fueling on PM mass emissions strongly depends on the engine load in a CRDI engine. They also suggested that use of biodiesel and increased fuel injection pressure effectively reduced PM emissions at low load conditions, however biodiesel did not show significant effect at moderate and higher engine loads on the overall PM mass emissions. Yehliu et al. [126] reported increased brake specific PM mass emissions with B100 vis-a-vis diesel at some operating conditions but reduction at other conditions for both single and split injection mode in a CRDI engine. At some operating conditions, increased PM mass was attributed to increase in SOF fraction of particulates due to relatively lower volatility of biodiesel in comparison to mineral diesel. Particulates from B100 mainly consisted of condensed organics, because particle number concentration dropped dramatically in comparison to particle number concentration for mineral diesel, when a thermo-denuder was set at 400 °C. instead of 30 °C, in order to remove organics [126]. Kegl [127] reported 50% reduction in smoke along with other regulated pollutants for a B100 fuelled bus engine at a retarded fuel injection timing (19° bTCD) in comparison to mineral diesel engine at a normal fuel injection timing (23° bTDC) in an European transient cycle (ESC) test-cycle (Fig. 15). 3.1.2. Effect of fuel composition After 1970’s energy crisis, many researchers started exploring a suitable alternative fuel for mineral diesel. Biodiesel was consid-

Fig. 15. Influence of injection pump timing and fuel on engine characteristics at peak torque [127].

324

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

ered as one of the potential alternative fuels for diesel. One major advantage of biodiesel is that it has inherent oxygen content in the fuel molecule itself, which enhances the probability of complete combustion. On the other hand, biodiesel’s evaporative properties are not as good as mineral diesel, therefore it has low evaporation rate at relatively lower engine loads, because of lower in-cylinder temperatures. Shi et al. [108] reported significant reduction in PM emissions from blend of ethanol:methyl soyate:diesel (5:20:75) compared to that from mineral diesel [108]. Zhu et al. [25] observed that PM emissions from biodiesel fuelled engine operation were lower than mineral diesel fuelled engine. PM emissions further reduced with an increasing ethanol/methanol concentration in biodiesel–alcohol blend at medium and high engine loads. PM reduction by addition of alcohol to biodiesel was due to higher oxygen content of alcohol–biodiesel blend compared to biodiesel, which improved combustion and reduced PM emissions. Also, alcohol in the blended fuel reduced the cetane number hence increased ignition delay period therefore higher fuel quantity burned in premixed combustion phase, resulting in lower PM emissions [25]. However, 15% alcohol blend leads to higher PM emissions than biodiesel (B100) and mineral diesel at low loads [25] (Fig. 16). Yoon et al. [128] observed significantly lower filter smoke number (FSN) for biodiesel–ethanol blend (90:10) in comparison to mineral diesel with double injection strategy. Lower soot emissions were primarily due to higher oxygen content of the blended fuel and absence of soot precursors (sulfur and aromatics contents) [128]. Particle number concentration of larger particles, which majorly contribute to PM mass for biodiesel–ethanol blend, were significantly lower than mineral diesel [128]. Lu et al. [129] reported that in their experiments of port injection of ethanol in a biodiesel fuelled engine, CO and HC emissions increased compared to biodiesel (B100) operated engine, and 35–85% reduction in NOx and smoke was observed. Kohoutek et al. [130] used ULSD and got 30% lower PM mass and number emissions in comparison to 410 ppm sulfur containing diesel in a modern DI diesel engine. At high engine load, specific emission of PM1.8 from biodiesel decreased by 68.4% and 50.3%, compared with LSD and ULSD, respectively [67]. PM mass emission reduced by 20.6% upon using biodiesel compared to LSD and was slightly on the higher side as compared to ULSD at lower engine loads. Dwivedi et al. [131] reported that total PM mass reduced by 20% with B20 in comparison to mineral diesel in a CIDI engine. Due to better lubricity of biodiesel, trace metal content in the particulate also reduced for B20. Benzene soluble organic fraction (BSOF) was found to be higher for B20. Kim and Choi [132] also found 20% lower PM mass for B20 fuelled engine in comparison to mineral diesel. 15% biodiesel and 5% ethanol blended with mineral diesel reduced PM mass further. Important observation was that total number of particles reduced for biodiesel but number of nuclei mode particles was higher for biodiesel compared to mineral diesel. Rakopoulos et al. [133] reported that peak value of smoke opacity reduced by 40% and 73%, respectively for the biodiesel and n-butanol blends during transient tests in a CI engine. Here the relative fuel-bound oxygen plays a dominant role. In a review, Graboski et al. [134] concluded that PM mass emission reduction was proportional to the fuel oxygen content as long as cetane number was higher than 45 or density was lower than 0.89 kg/l. Schönborn et al. [30] studied the effect of carbon chain length and degree of unsaturation on PM mass emissions. Total particulate mass emitted by mineral diesel was higher than biodiesel (Fig. 17). Lower particulate mass emission from biodiesel were attributed to higher fuel oxygen content, which helps in oxidation of soot particles and soot precursors. The behenic acid methyl ester (22 carbon atoms fatty acid chain) showed a distinctively higher emission of PM mass than other lower fatty acid chain length

Fig. 16. Variation of brake specific PM emissions with engine loads for different fuels [25].

Fig. 17. Effect of carbon chain length on total PM mass emissions (450 bar injection pressure, 4 bar IMEP, 7.1° bTDC SOI) [30].

molecules. It was possibly due to spray and fuel–air mixing being affected by fuel’s high viscosity and low volatility. 3.2. Particulate size-number distribution Most emission regulations globally prescribe PM mass measurement and control. Size affects the behavior of particulates in the engine as well as in the environment [22]. Adverse health effects due to particulates are more severe for smaller nuclei mode particulates. This fact has been recognized and new emission legislations, beyond Euro-5 also prescribe limit on total particle number concentration along with particulate mass. Table 5 shows typical contribution of different size particles emitted by diesel engines to PM mass and total numbers. Almost 90% of the particulates emitted from diesel engines originates as nuclei mode particles. Parihar et al. [89] suggested that contribution made by PM2.5 particles to the total PM mass varies from 75% to 95%, depending on engine load, for mineral diesel as well as B20. Particles larger than 10 lm contributed up to 10–15% to cumulative mass whereas particles in size range 2.5–10 lm contribute up to 20–30% of the total PM mass. Li et al. [29] reported that densities of particles smaller than 50 nm varied from 1.1 g/cm3 to 1.6 g/cm3. For particles having mobility diameter closer to 1 lm, particle densities varied from 0.2

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

325

Table 5 Size based classification of engine exhaust particulates [1,22].

Nuclei mode Accumulation mode Coarse mode

Size (nm)

% Number

% Mass

5–50 50–700 700–10,000

90 1–10 0–2

1–20 60–94 5–20

to 0.6 g/cm3. Hence, methods employed to control PM mass do not necessarily result in particulate number reduction. In this scenario, it is important to characterize the effect of biodiesel on particulate number emissions. Kawano et al. [135] reported that particle size distribution of both diesel and RME was mono-modal, and this distribution mostly covered accumulation mode particles for all the engine loads. The size distribution of accumulation mode particles of RME shifted toward smaller sizes compared to diesel, and peak position of particle size distribution was almost constant for varying engine loads. An increase in engine load increased the peak number concentration in case of diesel and reduced the peak concentration for RME [135] and these findings were in line with other studies which suggested that biodiesel particulates mostly comprise of SOF, which gets destroyed in engine in high temperature conditions existing at high loads. Raahede [136] deduced a decreasing trend for particle number concentration, when moving from reference diesel to biodiesel (B20) and this trend was attributed to higher fuel bound oxygen. Puzun et al. [137] studied the particle size-number distribution of Rapeseed biodiesel blends and mineral diesel in a high-pressure CRDI engine. They reported that particulate sizes emitted from this engine were mostly smaller than 300 nm [137]. For diesel, particle number concentrations showed single peak distributions dominating accumulation mode particles [137] (Fig. 18). For biodiesel blends at lower and intermediate loads, double peak in particle size-number distribution was seen with increasing concentration of biodiesel in the fuel and nuclei mode particle numbers increased significantly; particles with sizes more than 50 nm (accumulation mode) decreased and peak of number concentration shifted towards smaller particles [137]. Tan et al. [34] reported that peak value of nucleation mode particles below 30 nm increased with increasing biodiesel concentration in the fuel blend. Nucleation mode particle size peak becomes larger with increasing biodiesel blend ratio, and accumulation mode particle size peak value becomes smaller. Three mechanisms were thought to lead to greater nucleation mode particle formation: (i) high super-saturation encourages formation of new particles by nucleation due to scarcity of the solid soot surface, (ii) increased viscosity and lower volatility of biodiesel leads to formation of higher SOF, and (iii) oxygen content of biodiesel causes carbonaceous particle to transform from fine particles to ultra-fine particles or nanoparticles. Agarwal et al. [5] reported that particulate number emissions for B100 were higher than mineral diesel however, these were comparable for B20 and diesel at lower loads. Jung et al. [20] investigated the effect of biodiesel on oxidation of particulates using soy methyl ester (SME) and diesel (#2) at 1400 rpm engine speed and 75% load. Accumulation mode particle number concentration and particle volume distribution were lowered by 38% and 82%, respectively for SME. They also reported lower particle numbers than mineral diesel in larger particle size range above 50 nm, but very similar numbers was observed in nuclei region below 50 nm. This indicated towards lower PM mass emission from B100 because of lower soot pyrolysis due to presence of fuel oxygen. High numbers of engine out nuclei mode particles with B100 are seen due to presence of large number of condensed droplets of high boiling point hydrocarbons, which are also responsible

(a) Particle number concentrations and proportion in different modes

(b) Particle number concentrations and proportion in different modes Fig. 18. Euro IV diesel and biodiesel (BTL) fuel particle size distribution in different conditions [137].

for high VOF. They reported that rate of oxidation of SME particulates was 6 times faster than diesel particulates [20], which verified the above hypothesis.

3.2.1. Effect of injection strategy Most modern engines are now equipped with advanced fuel injection systems controlled by electronic control unit (ECU). These injection systems are designed in order to reduce total PM mass emissions. However, there is strong need to investigate the particle number emissions also for varying injection parameters because it is proved beyond a reasonable doubt that finer diesel particulates have higher toxicity given their ability to penetrate into the deepest part of human lungs. Several studies have been carried out to study the particle number emissions from different types of engines, fuels and at varying engine operating conditions and some of them are discussed in this section. Desantes et al. [138] investigated the effect of fuel injection pressure, start of injection (SOI) timings and exhaust gas recirculation (EGR) on engine exhaust particle size-number distribution from a heavy-duty diesel engine [138]. Increasing fuel injection pressure reduced number of accumulation mode particle and favored nuclei mode particle formation. Increasing fuel injection pressure improved air–fuel mixing, which reduced the spread of fuel-rich zones responsible for formation of carbonaceous soot particles. Reduction in the concentration of carbonaceous particles resulted in reduction in number of accu-

326

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

Fig. 20. Effect of carbon chain length on particulate size distribution (450 bar injection pressure, 4 bar IMEP, 7.1° bTDC SOI) [30].

Fig. 19. Effect of fuel injection pressure on number of nuclei mode particles (2–67 nm) for biodiesel blends [139].

mulation mode particles. Application of EGR suppressed the nucleation mode particles and increased number of accumulation mode particles. Advanced injection timing slightly reduced the number of accumulation mode particles without shifting the position of peak concentration [138]. Sinha et al. [139] reported increase in total number concentration of nuclei mode particles with increasing fuel injection pressure for diesel as well as biodiesel blends (Fig. 19). As biodiesel percentage increases in the blend, it increases fuel’s oxygen content, leading for formation of fewer number of carbonaceous accumulation mode particles. Since lower number of carbonaceous particles are available for adsorption of SOF, the partial pressure of these organic fractions increase, leading to higher numbers of nuclei mode particles. Also, biodiesel produces higher SOF , which adds to the partial pressure of gaseous hydrocarbons forming SOFs, further expediting nucleation process. Sinha et al. [139] also reported reduced accumulation mode particle numbers (50–1000 nm) at all fuel injection pressures except 600 bar from Soybean biodiesel (B100) vis-a-vis ULSD. 3.2.2. Effect of fuel composition Several studies showed that biodiesel and mineral diesel have different particle size-number distributions [25,30,140]. It is important from regulatory norms stand point to evaluate the particle number emissions for biodiesel. Pham et al. [140] reported that saturated short-chain length FAMEs reduce NOx and particulate number concentration, but led to higher BSFC as well as higher ROS emissions. Unsaturated FAMEs emit lower particulate and ROS, but higher NOx. Higher particulate mass (Fig. 10) for diesel originated from a larger number of accumulation mode particles (Fig. 12). Progressively higher number of nucleation mode particles was seen for fatty acid esters with longer fatty acid chains (Fig. 20). It is possible that the nuclei mode particles consist of high boiling point constituents of fuel which remains unburned and condenses in the exhaust gas [30]. Schönborn et al. also reported that increase in alcohol chain length from 1 to 2 carbon atoms reduced NOx emissions, but increased total PM mass emission, even under constant ignition delay and similar heat release conditions [30]. Zhu et al. [25] compared the particulate size-number distribution for biodiesel and biodiesel–alcohol blends with mineral diesel [25]. At all engine loads, total particulate number concentration from

biodiesel was higher than that from mineral diesel. Addition of ethanol/methanol in biodiesel reduced the total number concentration of particles dramatically below the level of mineral diesel fuelled engine operation [25]. They also suggested, like many other researchers, that blending alcohols with biodiesel decreases carbon content and increases oxygen content of the fuel, leading to reduction in number of nuclei mode particles and total particulate number concentration [25,141]. Lapuerta et al. [142] reported that higher unsaturation level of biodiesel led to retarded start of combustion, higher NOx emissions, higher heat release rate, lower smoke opacity, lower particulate mass emissions and lower particulate size distribution. Song et al. [36] reported that number of nucleation mode particles decreased or remained constant, and number of accumulation mode particles above 30 nm increased for oxidized biodiesel blends compared to non-oxidized biodiesel blend. The particulate mass-size distribution for oxidized biodiesel blend reduced by 5–23.4% compared to non-oxidized biodiesel blend at all engine loads. Total particle number concentration for oxidized biodiesel blend compared with non-oxidized biodiesel blend was found to be strongly dependent on the test conditions. Nuszkowski et al. [143] reported that the use of cetane improver additives resulted in lower number of particulates in entire particle size range measured. The reduced particle number concentrations were in the diameter ranges of 6–56 nm and 100–205 nm, respectively. Addition of biodiesel reduced the particle number concentration in the diameter range of 6–56 nm and 100–487 nm and was not affected in other size ranges during transient engine operation [143]. Sinha et al. [139] investigated the effect of using biodiesel on particulate emissions from a HSDI engine. They observed that particle number density increased and particle size-mass distribution decreased with increasing blending ratio of biodiesel. Zhang et al. [32] studied particle size-number distribution of diesel (1135 ppm sulfur) and biodiesel (64 ppm sulfur) blends. Number concentration of nuclei mode particles were three orders of magnitude higher for B60 and other lower biodiesel blends including mineral diesel in comparison to B100. They gave the hypothesis that in case of high sulfur containing fuel, hydrated H2SO4 nuclei acts as precursor for nuclei mode particulate formation (when the blend concentration was lower than B60). For B100, nuclei mode particulate concentration was low due to its extremely low sulfur content (64 ppm). Most of the studies included in the review of physical characterization of biodiesel particulate showed a clear and significant reduction in particulate mass emission from biodiesel fuelled diesel engines. However, higher viscosity and density result in

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

delayed atomization in the combustion chamber. Inherent oxygen content of biodiesel improves fuel’s combustion characteristics. Superior injection strategies such as multiple injections, and higher fuel injection pressures increase the applicability of biodiesel in diesel engines. Higher fuel injection pressures and multiple injections improve the atomization characteristics of biodiesel and compensate for the influence of higher viscosity of biodiesel.

4. Summary It is essential to evaluate the effect of biodiesel on the particulate emissions from diesel engines before it can be implemented on a large scale worldwide. Diesel particulate consists of EC/OC, trace metals, and organic compounds. Most of the diesel particles are nano-particles. Their composition varies and depends strongly upon engine operating conditions. A large number of researchers characterized biodiesel particles for their physical and chemical characteristics and these studies are summarized below: 1. Biodiesel has an advantage and emits lower particulate mass emissions compared to mineral diesel for most engine technologies and all engine load-speed conditions. Most studies suggested that there is a large reduction in total PM emissions by using biodiesel or biodiesel blends with mineral diesel. 20% blend of biodiesel with diesel showed good result in terms of lower particulate mass emission. B100 further reduced particulate emissions but not in the same proportion as that of B20. 2. Total particle number emissions also reduced with use of biodiesel but particle number emissions near the nano-size range were observed to be higher for biodiesel. 3. Biodiesel is an oxygenated fuel, and fuel oxygen helps in improving combustion inside the combustion chamber, resulting in lower PM mass emissions. Presence of fuel oxygen reduces pyrolysis reactions in the combustion chamber. Pyrolysis of fuel and lubricating oil in oxygen deficient regions of the combustion chamber is the main reason for particulate formation in the engine. 4. Biodiesel shows an order of magnitude lower OC content of the particulate emitted compared to mineral diesel. No significant reductions were observed for EC content of the particulate. For biodiesel, most studies reported higher EC/OC ratio along with lower total PM mass emissions, which suggests its lower environmental toxicity compared to mineral diesel. 5. For biodiesel, generally PAHs emission were found to be relatively lower but some of the specific PAHs adsorbed by particulate were slightly higher compared to mineral diesel. This indicates toward the presence of structurally strong PAHs in biodiesel. Possibly these PAHs are difficult to oxidize in the engine combustion chamber, even under higher temperature and pressure conditions. 6. Biodiesel helps particulate oxidation in DOC/DPF. Biodiesel inherently contains some trace metals, which possibly act as catalysts in the after-treatment devices and lower particulate emissions. Lower particulate emissions from biodiesel also result in a longer useful life of after-treatment devices. Overall, biodiesel emits relatively lower PM mass emissions which have lesser environmental and health related toxicity, and impacts the exhaust gas after-treatment devices life positively, in addition to protecting the environment. In future research efforts to estimate the effect of biodiesel on particulate emissions, concrete experimental evidence on differences in toxic potential between diesel and biodiesel particulates need to be assessed. Transformation of biodiesel particulates in the atmosphere with time needs to be investigated thoroughly. Correlation between

327

different chemical compositions (fatty acid profiles) of biodiesels along with different fuel injection strategies on PM mass, number-size distribution and particulate toxicity also needs more comprehensive investigations.

References [1] Zielinska B. Atmospheric transformation of diesel emissions. Exp Toxicol Pathol 2005;57:31–42. [2] Schroeder O, Krahl J, Bünger J. Environmental and health effects caused by the use of biodiesel. SAE Technical Paper 1999-01-3561; 1999. [3] Krahl J, Munack A, Schröder O, Stein H, Bünger J. Influence of biodiesel and different designed diesel fuels on the exhaust gas emissions and health effects. SAE Technical Paper 2003-01-3199; 2003. [4] Krahl J, Munack A, Ruschel Y, Schröder O, Bünger J. Exhaust gas emissions and mutagenic effects of diesel fuel, biodiesel and biodiesel blends. SAE Technical Paper 2008-01-2508; 2008. [5] Agarwal AK, Gupta T, Kothari A. Particulate emissions from biodiesel vs diesel fuelled compression ignition engine. Renew Sustain Energy Rev 2011;15:3278–300. [6] Agarwal AK, Gupta T, Dixit N, Shukla PC. Assessment of toxic potential of primary and secondary particulates/aerosols from biodiesel vis-a-vis mineral diesel fuelled engine. Inhalation Toxicol 2013;25:325–32. [7] Agarwal AK, Gupta T, Kothari A. Toxic potential evaluation of particulate matter emitted from a constant speed compression ignition engine: a comparison between straight vegetable oil and mineral diesel. Aerosol Sci Technol 2010;44:724–33. [8] Mohsin R, Majid Z, Shihnan A, Nasri N, Sharer Z. Effect of biodiesel blends on engine performance and exhaust emission for diesel dual fuel engine. Energy Convers Manage 2014;88:821–8. [9] Rashedul H, Masjuki H, Kalam M, Ashraful A, Rahman SA, Shahir S. The effect of additives on properties, performance and emission of biodiesel fuelled compression ignition engine. Energy Convers Manage 2014;88:348–64. [10] López I, Quintana C, Ruiz J, Cruz-Peragón F, Dorado M. Effect of the use of olive– pomace oil biodiesel/diesel fuel blends in a compression ignition engine: preliminary exergy analysis. Energy Convers Manage 2014;85:227–33. [11] Roy MM, Wang W, Alawi M. Performance and emissions of a diesel engine fueled by biodiesel–diesel, biodiesel–diesel-additive and kerosene–biodiesel blends. Energy Convers Manage 2014;84:164–73. [12] Nizah MR, Taufiq-Yap Y, Rashid U, Teo SH, Nur ZS, Islam A. Production of biodiesel from non-edible Jatropha curcas oil via transesterification using Bi2O3–La2O3 catalyst. Energy Convers Manage 2014;88:1257–62. [13] Rizwanul Fattah I, Masjuki H, Kalam M, Mofijur M, Abedin M. Effect of antioxidant on the performance and emission characteristics of a diesel engine fueled with palm biodiesel blend. Energy Convers Manage 2014;79:265–72. [14] Mohan B, Yang W, Tay KL, Yu W. Experimental study of spray characteristics of biodiesel derived from waste cooking oil. Energy Convers Manage 2014;88:622–32. [15] Flynn PF, Durrett RP, Hunter GL, zur Loye AO, Akinyemi O, Dec JE, et al. Diesel combustion: an integrated view combining laser diagnostics, chemical kinetics, and empirical validation. SAE Technical Paper 1999-01-0509; 1999. [16] Dhar A, Agarwal AK. Effect of multiple injections on particulate size-number distributions in a common rail direct injection engine fueled with karanja biodiesel blends. SAE Technical Paper 2013-01-1554; 2013. [17] Wang X, Huang Z, Kuti OA, Zhang W, Nishida K. Experimental and analytical study on biodiesel and diesel spray characteristics under ultra-high injection pressure. Int J Heat Fluid Flow 2010;31:659–66. [18] Gamble JF, Nicolich MJ, Boffetta P. Lung cancer and diesel exhaust: an updated critical review of the occupational epidemiology literature. Crit Rev Toxicol 2012;42:549–98. [19] Heywood JB. Internal combustion engine fundamentals. New York: Mcgrawhill; 1988. [20] Jung H, Kittelson DB, Zachariah MR. Characteristics of SME biodiesel-fueled diesel particle emissions and the kinetics of oxidation. Environ Sci Technol 2006;40:4949–55. [21] Eastwood P. Particulate emissions from vehicles. John Wiley & Sons; 2008. [22] Kittelson DB. Engines and nanoparticles: a review. J Aerosol Sci 1998;29:575–88. [23] Stevenson R. The morphology and crystallography of diesel particulate emissions. Carbon 1982;20:359–65. [24] Rounce P, Tsolakis A, York A. Speciation of particulate matter and hydrocarbon emissions from biodiesel combustion and its reduction by aftertreatment. Fuel 2012;96:90–9. [25] Zhu L, Cheung C, Zhang W, Huang Z. Emissions characteristics of a diesel engine operating on biodiesel and biodiesel blended with ethanol and methanol. Sci Total Environ 2010;408:914–21. [26] Williams P, Abbass M, Andrews G, Bartle K. Diesel particulate emissions: the role of unburned fuel. Combust Flame 1989;75:1–24. [27] Chuepeng S, Xu H, Tsolakis A, Wyszynski M, Price P, Stone R, et al. Particulate emissions from a common rail fuel injection diesel engine with RME-based biodiesel blended fuelling using thermo-gravimetric analysis. SAE Technical Paper 2008-01-0074; 2008.

328

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

[28] Young L-H, Liou Y-J, Cheng M-T, Lu J-H, Yang H-H, Tsai YI, et al. Effects of biodiesel, engine load and diesel particulate filter on nonvolatile particle number size distributions in heavy-duty diesel engine exhaust. J Hazard Mater 2012;199:282–9. [29] Li H, Lea-Langton A, Andrews G, Thompson M, Musungo C. Comparison of exhaust emissions and particulate size distribution for diesel, biodiesel and cooking oil from a heavy duty DI diesel engine. SAE Technical Paper 2008-010076; 2008. [30] Schönborn A, Ladommatos N, Allan R, Williams J, Rogerson J. Effect of the molecular structure of individual fatty acid alcohol esters (biodiesel) on the formation of NOx and particulate matter in the diesel combustion process. SAE Int J Fuels Lubricants 2009;1:849–72. [31] Lapuerta M, Armas O, Rodriguez-Fernandez J. Effect of biodiesel fuels on diesel engine emissions. Prog Energy Combust Sci 2008;34:198–223. [32] Zhang X, Wang H, Li L, Wu Z. Characteristics of particulates and exhaust gases emissions of DI diesel engine employing common rail fuel system fueled with bio-diesel blends. SAE Technical Paper 2008-01-1834; 2008. [33] Soewono A, Rogak S. Morphology and microstructure of engine-emitted particulates. SAE Technical Paper 2009-01-1906; 2009. [34] Tan P-Q, Lou D-M, Hu Z-Y. Nucleation mode particle emissions from a diesel engine with biodiesel and petroleum diesel fuels. SAE Technical Paper 201001-0787; 2010. [35] Tinsdale M, Price P, Chen R. The impact of biodiesel on particle number, size and mass emissions from a Euro4 diesel vehicle. SAE Int J Eng 2010;3: 597–608. [36] Song H, Lee MH, Kang H, Kim K. The effects of oxidation deterioration biodiesel on particulate emission from heavy duty diesel engine. SAE Technical Paper 2012-01-1600; 2012. [37] Poitras M-J, Rosenblatt D, Chan TW, Rideout G. Impact of varying biodiesel blends on direct-injection light-duty diesel engine emissions. SAE Technical Paper 2012-01-1313; 2012. [38] Gangwar JN, Gupta T, Agarwal AK. Composition and comparative toxicity of particulate matter emitted from a diesel and biodiesel fuelled CRDI engine. Atmos Environ 2012;46:472–81. [39] Schauer J, Christensen C, Kittelson D, Johnson J, Watts W. Impact of ambient temperatures and driving conditions on the chemical composition of particulate matter emissions from non-smoking gasoline-powered motor vehicles. Aerosol Sci Technol 2008;42:210–23. [40] Pakbin P, Ning Z, Schauer JJ, Sioutas C. Characterization of particle bound organic carbon from diesel vehicles equipped with advanced emission control technologies. Environ Sci Technol 2009;43:4679–86. [41] Liu YY, Lin TC, Wang YJ, Ho WL. Carbonyl compounds and toxicity assessments of emissions from a diesel engine running on biodiesels. J Air Waste Manage Assoc 2009;59:163–71. [42] Limbach LK, Wick P, Manser P, Grass RN, Bruinink A, Stark WJ. Exposure of engineered nanoparticles to human lung epithelial cells: influence of chemical composition and catalytic activity on oxidative stress. Environ Sci Technol 2007;41:4158–63. [43] Donaldson K, Tran L, Jimenez LA, Duffin R, Newby DE, Mills N, et al. Combustion-derived nanoparticles: a review of their toxicology following inhalation exposure. Part Fibre Toxicol 2005;2:10. [44] Künzli N, Mudway IS, Götschi T, Shi T, Kelly FJ, Cook S, et al. Comparison of oxidative properties, light absorbance, and total and elemental mass concentration of ambient PM2. 5 collected at 20 European sites. Environ Health Perspect 2006;114:684. [45] Baulig A, Garlatti M, Bonvallot V, Marchand A, Barouki R, Marano F, et al. Involvement of reactive oxygen species in the metabolic pathways triggered by diesel exhaust particles in human airway epithelial cells. Am J PhysiolLung Cell Mol Physiol 2003;285:L671–9. [46] González-Flecha B. Oxidant mechanisms in response to ambient air particles. Mol Aspects Med 2004;25:169–82. [47] Pillay A, Elkadi M, Fok S, Stephen S, Manuel J, Khan M, et al. A comparison of trace metal profiles of neem biodiesel and commercial biofuels using high performance ICP-MS. Fuel 2012;97:385–9. [48] Betha R, Balasubramanian R. Emissions of particulate-bound elements from stationary diesel engine: characterization and risk assessment. Atmos Environ 2011;45:5273–81. [49] Carrales M, Martin RW. Sulfur content of crude oils. Bureau of Mines: US Department of the Interior; 1975. [50] Ravindra K, Sokhi R, Van Grieken R. Atmospheric polycyclic aromatic hydrocarbons: source attribution, emission factors and regulation. Atmos Environ 2008;42:2895–921. [51] McDonald JD, Barr EB, White RK, Chow JC, Schauer JJ, Zielinska B, et al. Generation and characterization of four dilutions of diesel engine exhaust for a subchronic inhalation study. Environ Sci Technol 2004;38:2513–22. [52] Schauer JJ, Kleeman MJ, Cass GR, Simoneit BR. Measurement of emissions from air pollution sources. 2. C1 through C30 organic compounds from medium duty diesel trucks. Environ Sci Technol 1999;33:1578–87. [53] Schauer JJ, Kleeman MJ, Cass GR, Simoneit BR. Measurement of emissions from air pollution sources. 5. C1–C32 organic compounds from gasolinepowered motor vehicles. Environ Sci Technol 2002;36:1169–80. [54] Grosjean D, Grosjean E, Gertler AW. On-road emissions of carbonyls from light-duty and heavy-duty vehicles. Environ Sci Technol 2001;35:45–53. [55] Kristensson A, Johansson C, Westerholm R, Swietlicki E, Gidhagen L, Wideqvist U, et al. Real-world traffic emission factors of gases and particles

[56]

[57]

[58]

[59]

[60]

[61]

[62]

[63]

[64] [65] [66]

[67]

[68]

[69]

[70] [71]

[72]

[73] [74]

[75]

[76]

[77]

[78]

[79] [80]

[81] [82]

[83]

measured in a road tunnel in Stockholm, Sweden. Atmos Environ 2004;38:657–73. Loeffler KW, Koehler CA, Paul NM, De Haan DO. Oligomer formation in evaporating aqueous glyoxal and methyl glyoxal solutions. Environ Sci Technol 2006;40:6318–23. Madden MC, Dailey LA, Stonehuerner JG, Harris DB. Responses of cultured human airways epithelial cells treated with diesel exhaust extracts will vary with the engine load. J Toxicol Environ Health A 2003;66:2281–97. Pang X, Shi X, Mu Y, He H, Shuai S, Chen H, et al. Characteristics of carbonyl compounds emission from a diesel-engine using biodiesel–ethanol–diesel as fuel. Atmos Environ 2006;40:7057–65. Ho SSH, Ho KF, Lee SC, Cheng Y, Yu JZ, Lam KM, et al. Carbonyl emissions from vehicular exhausts sources in Hong Kong. J Air Waste Manag Assoc 2012;62:221–34. Cheung C, Zhu L, Huang Z. Regulated and unregulated emissions from a diesel engine fueled with biodiesel and biodiesel blended with methanol. Atmos Environ 2009;43:4865–72. Di Y, Cheung C, Huang Z. Experimental investigation on regulated and unregulated emissions of a diesel engine fueled with ultra-low sulfur diesel fuel blended with biodiesel from waste cooking oil. Sci Total Environ 2009;407:835–46. Takada K, Yoshimura F, Ohga Y, Kusaka J, Daisho Y. Experimental study on unregulated emission characteristics of turbocharged DI diesel engine with common rail fuel injection system. SAE Technical Paper 2003-01-3158; 2003. Ballesteros R, Hernandez J, Lyons L, Cabanas B, Tapia A. Speciation of the semivolatile hydrocarbon engine emissions from sunflower biodiesel. Fuel 2008;87:1835–43. Machado Corrêa S, Arbilla G. Carbonyl emissions in diesel and biodiesel exhaust. Atmos Environ 2008;42:769–75. Xue J, Grift TE, Hansen AC. Effect of biodiesel on engine performances and emissions. Renew Sustain Energy Rev 2011;15:1098–116. Lea-Langton A, Li H, Andrews GE. Comparison of particulate PAH emissions for diesel, biodiesel and cooking oil using a heavy duty DI diesel engine. SAE Technical Paper 2008-01-1811; 2008. Lu T, Huang Z, Cheung C, Ma J. Size distribution of EC, OC and particle-phase PAHs emissions from a diesel engine fueled with three fuels. Sci Total Environ 2012;438:33–41. Karavalakis G, Stournas S, Bakeas E. Effects of diesel/biodiesel blends on regulated and unregulated pollutants from a passenger vehicle operated over the European and the Athens driving cycles. Atmos Environ 2009;43:1745–52. Pan J, Quarderer S, Smeal T, Sharp C. Comparison of PAH and nitro-PAH emissions among standard diesel fuel, biodiesel fuel, and their blend on diesel engines. In: Proceedings of the 48th ASMS conference on mass spectrometry and allied topics, Long Beach, CA; 2000. Nisbet IC, LaGoy PK. Toxic equivalency factors (TEFs) for polycyclic aromatic hydrocarbons (PAHs). Regul Toxicol Pharmacol 1992;16:290–300. Karavalakis G, Fontaras G, Ampatzoglou D, Kousoulidou M, Stournas S, Samaras Z, et al. Effects of low concentration biodiesel blends application on modern passenger cars. Part 3: Impact on PAH, nitro-PAH, and oxy-PAH emissions. Environ Pollut 2010;158:1584–94. Bakeas E, Karavalakis G, Fontaras G, Stournas S. An experimental study on the impact of biodiesel origin on the regulated and PAH emissions from a Euro 4 light-duty vehicle. Fuel 2011;90:3200–8. Rhead M, Hardy S. The sources of polycyclic aromatic compounds in diesel engine emissions. Fuel 2003;82:385–93. Richter H, Howard J. Formation of polycyclic aromatic hydrocarbons and their growth to soot—a review of chemical reaction pathways. Prog Energy Combust Sci 2000;26:565–608. Riddle SG, Jakober CA, Robert MA, Cahill TM, Charles MJ, Kleeman MJ. Large PAHs detected in fine particulate matter emitted from light-duty gasoline vehicles. Atmos Environ 2007;41:8658–68. Miet K, Le Menach K, Flaud P-M, Budzinski H, Villenave E. Heterogeneous reactivity of pyrene and 1-nitropyrene with NO2: kinetics, product yields and mechanism. Atmos Environ 2009;43:837–43. Heeb NV, Schmid P, Kohler M, Gujer E, Zennegg M, Wenger D, et al. Secondary effects of catalytic diesel particulate filters: conversion of PAHs versus formation of nitro-PAHs. Environ Sci Technol 2008;42:3773–9. Bagley ST, Gratz LD, Johnson JH, McDonald JF. Effects of an oxidation catalytic converter and a biodiesel fuel on the chemical, mutagenic, and particle size characteristics of emissions from a diesel engine. Environ Sci Technol 1998;32:1183–91. Tan P-Q, Hu Z-Y, Lou D-M. Regulated and unregulated emissions from a lightduty diesel engine with different sulfur content fuels. Fuel 2009;88:1086–91. Cheung C, Di Y, Huang Z. Experimental investigation of regulated and unregulated emissions from a diesel engine fueled with ultralow-sulfur diesel fuel blended with ethanol and dodecanol. Atmos Environ 2008;42:8843–51. Zervas E. Regulated and non-regulated pollutants emitted from two aliphatic and a commercial diesel fuel. Fuel 2008;87:1141–7. Shukla PC, Gupta T, Agarwal AK. A comparative morphological study of primary and aged particles emitted from a biodiesel (B20) vis-à-vis diesel fuelled CRDI engine. Aerosol Air Qual Res 2014;14:934–42. Shi X, He K, Song W, Wang X, Tan J. Effects of a diesel oxidation catalyst on gaseous pollutants and fine particles from an engine operating on diesel and biodiesel. Front Environ Sci Eng 2012;6:463–9.

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330 [84] Zhu L, Cheung C, Zhang W, Fang J, Huang Z. Effects of ethanol–biodiesel blends and diesel oxidation catalyst (DOC) on particulate and unregulated emissions. Fuel 2013;113:690–6. [85] Williams A, McCormick R, Luecke J, Brezny R, Geisselmann A, Voss K, et al. Impact of biodiesel impurities on the performance and durability of DOC, DPF and SCR technologies. SAE Int J Fuels Lubricants 2011;4:110–24. [86] Vertin K, He S, Heibel A. Impacts of B20 biodiesel on cordierite diesel particulate filter performance. SAE Technical Paper 2009-01-2736; 2009. [87] Austin G, Naber J, Johnson J, Hutton C. Effects of biodiesel blends on particulate matter oxidation in a catalyzed particulate filter during active regeneration. SAE Int J Fuels Lubricants 2010;3:194–218. [88] Asti M, Borla EM, Parussa F, Scpa CRF. Biodiesel Influence on particulate matter behavior during active and passive DPF regeneration. SAE Technical Paper 2011-24-0204; 2011. [89] Parihar A, Sethi V, Parikh P, Radhakrishna D. Physical characterization of particulate emissions from compression ignition engine operating on diesel and biodiesel fuels. SAE Technical Paper 2011-26-0026; 2011. [90] Di Iorio S, Beatrice C, Guido C, Napolitano P, Vassallo A, Ciaravino C. Impact of biodiesel on particle emissions and DPF regeneration management in a Euro5 automotive diesel engine. SAE Technical Paper 2012-01-0839; 2012. [91] Pidgeon J, Johnson J, Naber J. An experimental investigation into particulate matter oxidation in a catalyzed particulate filter with biodiesel blends on an engine during active regeneration. SAE Technical Paper 2013-01-0521; 2013. [92] EPA. A comprehensive analysis of biodiesel impacts on exhaust emissions. Assessment and Standards Division (Office of Transportation and Air Quality of the US Environmental Protection Agency). Report EPA 420-P-02-001; 2002. [93] Turrio-Baldassarri L, Battistelli CL, Conti L, Crebelli R, De Berardis B, Iamiceli AL, et al. Emission comparison of urban bus engine fueled with diesel oil and ‘biodiesel’blend. Sci Total Environ 2004;327:147–62. [94] Lin Y-f, Wu Y-pG, Chang C-T. Combustion characteristics of waste-oil produced biodiesel/diesel fuel blends. Fuel 2007;86:1772–80. [95] Nabi MN, Shahadat MMZ, Rahman MS, Beg MRA. Behavior of diesel combustion and exhaust emission with neat diesel fuel and diesel-biodiesel blends. SAE Technical Paper 2004-01-3034; 2004. [96] Alam M, Song J, Acharya R, Boehman A, Miller K. Combustion and emissions performance of low sulfur, ultra low sulfur and biodiesel blends in a DI diesel engine. SAE Technical Paper 2004-01-3024; 2004. [97] McGill R, Storey J, Wagner R, Irick D, Aakko P, Westerholm M, et al. Emission performance of selected biodiesel fuels. SAE Technical Paper 2003-01-1866; 2003. [98] Sharp CA, Ryan TW, Knothe G. Heavy-duty diesel engine emissions tests using special biodiesel fuels. SAE Technical Paper 2005-01-3671; 2005. [99] Spessert BM, Arendt I, Schleicher A. Influence of RME and vegetable oils on exhaust gas and noise emissions of small industrial diesel engines. SAE Technical Paper 2004-32-0070; 2004. [100] Wang W, Lyons D, Clark N, Gautam M, Norton P. Emissions from nine heavy trucks fueled by diesel and biodiesel blend without engine modification. Environ Sci Technol 2000;34:933–9. [101] Canakci M. Combustion characteristics of a turbocharged DI compression ignition engine fueled with petroleum diesel fuels and biodiesel. Bioresour Technol 2007;98:1167–75. [102] Canakci M, Van Gerpen JH. Comparison of engine performance and emissions for petroleum diesel fuel, yellow grease biodiesel, and soybean oil biodiesel. Trans ASAE 2003;46:937–44. [103] Qi D, Geng L, Chen H, Bian Y, Liu J, Ren X. Combustion and performance evaluation of a diesel engine fueled with biodiesel produced from soybean crude oil. Renewable Energy 2009;34:2706–13. [104] Kousoulidou M, Fontaras G, Ntziachristos L, Samaras Z. Biodiesel blend effects on common-rail diesel combustion and emissions. Fuel 2010;89:3442–9. [105] Krahl J, Munack A, Schröder O, Stein H, Herbst L, Kaufmann A, et al. The influence of fuel design on the exhaust gas emissions and health effects. SAE Technical Paper 2005-01-3772; 2005. [106] Grimaldi CN, Postrioti L, Battistoni M, Millo F. Common rail HSDI diesel engine combustion and emissions with fossil/bio-derived fuel blends. SAE Technical Paper 2002-01-0865; 2002. [107] Raheman H, Phadatare A. Diesel engine emissions and performance from blends of karanja methyl ester and diesel. Biomass Bioenergy 2004;27: 393–7. [108] Shi X, Pang X, Mu Y, He H, Shuai S, Wang J, et al. Emission reduction potential of using ethanol–biodiesel–diesel fuel blend on a heavy-duty diesel engine. Atmos Environ 2006;40:2567–74. [109] Xiaoming L, Yunshan G, Sijin W, Xiukun H. An experimental investigation on combustion and emissions characteristics of turbocharged DI engines fueled with blends of biodiesel. SAE Technical Paper 2005-01-2199; 2005. [110] McCormick RL, Tennant C, Hayes RR, Black S, Ireland J, McDaniel T, et al. Regulated emissions from biodiesel tested in heavy-duty engines meeting 2004 emission standards. SAE Technical Paper 2005-01-2200; 2005. [111] Verhaeven E, Pelkmans L, Govaerts L, Lamers R, Theunissen F. Results of demonstration and evaluation projects of biodiesel from rapeseed and used frying oil on light and heavy duty vehicles. SAE Technical Paper 2005-012201; 2005.

329

[112] Agarwal AK, Das L. Biodiesel development and characterization for use as a fuel in compression ignition engines. J Eng Gas Turbines Power 2001;123:440–7. [113] Qi D, Chen H, Geng L, Bian Y. Experimental studies on the combustion characteristics and performance of a direct injection engine fueled with biodiesel/diesel blends. Energy Convers Manage 2010;51:2985–92. [114] Lin Y-C, Lee W-J, Wu T-S, Wang C-T. Comparison of PAH and regulated harmful matter emissions from biodiesel blends and paraffinic fuel blends on engine accumulated mileage test. Fuel 2006;85:2516–23. [115] Robbins C, Hoekman SK, Ceniceros E, Natarajan M. Effects of biodiesel fuels upon criteria emissions. SAE Technical Paper 2011-01-1943; 2011. [116] Saanum I, Bysveen M, Hustad JE. Study of particulate matter-, NOx-and hydrocarbon emissions from a diesel engine fueled with diesel oil and biodiesel with fumigation of hydrogen, methane and propane. SAE Technical Paper 2008-01-1809; 2008. [117] Agarwal AK, Chaudhury V, Shukla PC. Macroscopic spray parameters of karanja oil and blends: a comparative study. SAE Technical Paper 2012-280028; 2012. [118] Suryawanshi JG, Deshpande N. Effect of injection timing retard on emissions and performance of a Pongamia oil methyl ester fuelled CI engine. SAE Technical Paper 2005-01-3677; 2005. [119] Corgard DD, Reitz RD. Effects of alternative fuels and intake port geometry on HSDI diesel engine performance and emissions. SAE Technical Paper 200101-0647, 2001. [120] Reitz R. Controlling DI diesel engine emissions using multiple injections and EGR. Combust Sci Technol 1998;138:257–78. [121] Uludogan A, Xin J, Reitz R. Exploring the use of multiple injectors and split injection to reduce DI diesel engine emissions. SAE Technical Paper 962058; 1996. [122] Han Z, Uludogan A, Hampson GJ, Reitz RD. Mechanism of soot and NOx emission reduction using multiple-injection in a diesel engine. SAE Technical Paper 960633; 1996. [123] Dec JE, Espey C. Ignition and early soot formation in a DI diesel engine using multiple 2-D imaging diagnostics. Presented at the society of automotive engineers international congress and exposition, Detroit, MI, 27 February–2 March, 1995. [124] Yamane K, Ueta A, Shimamoto Y. Influence of physical and chemical properties of biodiesel fuels on injection, combustin and exhaust emission characteristics in a direct injection compression ignition engine. Int J Engine Res 2001;2:249–61. [125] Ye P, Boehman AL. An investigation of the impact of injection strategy and biodiesel on engine NOx and particulate matter emissions with a commonrail turbocharged DI diesel engine. Fuel 2012;97:476–88. [126] Yehliu K, Boehman AL, Armas O. Emissions from different alternative diesel fuels operating with single and split fuel injection. Fuel 2010;89:423–37. [127] Kegl B. Effects of biodiesel on emissions of a bus diesel engine. Bioresour Technol 2008;99:863–73. [128] Yoon SH, Hwang JW, Lee CS. Effect of injection strategy on the combustion and exhaust emission characteristics of a biodiesel–ethanol blend in a di diesel engine. J Eng Gas Turbines Power 2010;132. [129] Lu X, Ma J, Ji L, Huang Z. Simultaneous reduction of NOx emission and smoke opacity of biodiesel-fueled engines by port injection of ethanol. Fuel 2008;87:1289–96. [130] Kohoutek P, Metz N, Richter K, Volkswagen A, AG M. Particulate matter emissions from modern diesel engines, impact of fuel quality and PMdevelopment from road transport in Germany. ILSI-Symposium vom; 1999. [131] Dwivedi D, Agarwal AK, Sharma M. Particulate emission characterization of a biodiesel vs diesel-fuelled compression ignition transport engine: a comparative study. Atmos Environ 2006;40:5586–95. [132] Kim H, Choi B. The effect of biodiesel and bioethanol blended diesel fuel on nanoparticles and exhaust emissions from CRDI diesel engine. Renewable Energy 2010;35:157–63. [133] Rakopoulos CD, Dimaratos AM, Giakoumis EG, Rakopoulos DC. Investigating the emissions during acceleration of a turbocharged diesel engine operating with bio-diesel or n-butanol diesel fuel blends. Energy 2010;35:5173–84. [134] Graboski M, McCormick R, Alleman T, Herring A. The effect of biodiesel composition on engine emissions from a DDC series 60 diesel engine. National Renewable Energy Laboratory (Report No: NREL/SR-510-31461); 2003. [135] Kawano D, Ishii H, Goto Y, Noda A. Application of biodiesel fuel to modern diesel engine. SAE Technical Paper 2006-01-0233; 2006. [136] Raahede C. Particle characteristics – PAH and gaseous emission from light duty CI engines fueled with biodiesel and biodiesel blends. SAE Technical Paper 2010-01-1277; 2010. [137] Puzun A, Wanchen S, Guoliang L, Manzhi T, Chunjie L, Shibao C. Characteristics of particle size distributions about emissions in a commonrail diesel engine with biodiesel blends. Procedia Environ Sci 2011;11:1371–8. [138] Desantes J, Bermúdez V, García J, Fuentes E. Effects of current engine strategies on the exhaust aerosol particle size distribution from a heavy-duty diesel engine. J Aerosol Sci 2005;36:1251–76. [139] Sinha A, Chandrasekharan J, Nargunde J, Bryzik W, Acharya K, Henein N. Effect of biodiesel and its blends on particulate emissions from HSDI diesel engine. SAE Technical Paper 2010-01-0798; 2010.

330

A.K. Agarwal et al. / Energy Conversion and Management 94 (2015) 311–330

[140] Pham P, Bodisco T, Stevanovic S, Rahman M, Wang H, Ristovski Z, et al. Engine performance characteristics for biodiesels of different degrees of saturation and carbon chain lengths. SAE Int J Fuels Lubricants 2013;6:188–98. [141] Di Y, Cheung C, Huang Z. Comparison of the effect of biodiesel–diesel and ethanol–diesel on the particulate emissions of a direct injection diesel engine. Aerosol Sci Technol 2009;43:455–65.

[142] Lapuerta M, Armas O, Rodríguez-Fernández J. Effect of the degree of unsaturation of biodiesel fuels on NOx and particulate emissions. SAE Int J Fuels Lubricants 2009;1:1150–8. [143] Nuszkowski J, Flaim K, Thompson G. The effect of cetane improvers and biodiesel on diesel particulate matter size. SAE Int J Fuels Lubricants 2011;4:23–33.