RSC Advances This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. This Accepted Manuscript will be replaced by the edited, formatted and paginated article as soon as this is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/advances Page 1 of 60 RSC Advances Stability of biodiesel, its improvement and the effect of antioxidant treated blends on engine performance and emission. M.M. Rashed1, M.A. Kalam2, H.H. Masjuki, H.K. Rashedul, A.M. Ashraful, I. Shancita, A.M. Ruhul t p i r Center for Energy Sciences, Department of Mechanical Engineering, Faculty of Engineering, c s University of Malaya u n a Abstract M Biodiesel consists of long chain fatty acid esters derived from vegetable oils, animal fats, and used d e oils. Biodiesel contains different types, amounts, and configurations of unsaturated fatty acids, t p which are prone to oxidation. Biodiesel stability is affected by its interaction with atmospheric e c oxygen, light, temperature, storage condition, and factors causing sediment formation. It can be c A classified broadly into three types: oxidation stability, thermal stability, and storage stability. s e Oxidative degradation occurs in biodiesel upon aerobic contact during storage, as well as contact c n with metal contaminants. Thermal instability focuses on the oxidation rate at higher temperatures, a v which is characterized by the formation of insolubles and increase in the weight of oil and fat. d A Storage stability is concerned with interaction between the physical and chemical characteristics C of biodiesel with environmental factors, such as light, metal contamination, color changes, and S R sediment formation. Antioxidant concentration greatly influences engine performance and emission. The BSFC of biodiesel fuel with antioxidant is less than that of fuel without antioxidant. 1 Corresponding author. M.M. Rashed, Department of Mechanical Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia. Tel.: +603 79674448; Fax: +603 79675317 E-mail: [email protected] 2 M.A. Kalam, Department of Mechanical Engineering, University of Malaya, 50603, Kuala Lumpur E-mail: [email protected] RSC Advances Page 2 of 60 Moreover, an antioxidant can significantly reduce NO formation during engine operation. Among x the available synthetic antioxidants, only three antioxidants (TBHQ, PY, and PG) can significantly increase biodiesel stability. This article presents an overview of the stability of biodiesel, including the methods available for the prediction of its different stability properties. Feasible remedies to improve the stability of biodiesel and the effect of antioxidants in stabilized blends on engine t p performance and emission are also discussed. i r c . s u n Keywords: Biodiesel, Oxidation stability, Thermal stability, Storage stability, Performance, a Emission, Antioxidant M Nomenclature d ASTM American standard test method e AV Acid value t p AOM Active oxygen method e APE Allylic position equivalent c AH Antioxidant c AACC American association of central A chemists BHA Butylated hydroxyanisole s BHT Butylated hydroxytoluene e BAPE Bis-allylic position equivalent c BP Break power n BSFC Break fuel specific fuel consumption a CO Carbone monoxide v CA Citric acid and caffeic acid d DI Direct injection engine A DPF Diesel particulate filter D Density C DSC Differential scanning calorimetry S EDTA Ethylenediamine tetraacetic acid R EHN Ethylhexyl nitrate FAME Fatty acid methyl ester FFA Free fatty acid HC Hydro carbon IV Iodine value IP Induction period NO Nitrogen oxides x OT Oxidation temperature OX oxidizability Page 3 of 60 RSC Advances OSI Oil stability index PDSC Pressure differential scanning calorimetry PAO polyalpholefins PY Pyrogallol PG Propyl gallate PV Peroxide value RIP Rancimat induction period TAN Total acid value t p TGA Thermogravity analysis TBHQ Tert-butylhydroquinone i r TDI Terbocharged direct injection c THC Total hydro carbon s V viscosity u n a M 1. Introduction d Biodiesel is defined as a vegetable oil- or animal fat-based diesel fuel consisting of long chain e t p alkyl esters. Biodiesel is produced by chemically reacting lipids (e.g., vegetable oil, animal fat) e c with an alcohol to produced fatty acid esters 1, 2. The fatty acid profile of biodiesel corresponds to c A that of parent oil or fat, which is a key factor that influences its fuel characteristics. The stability s of fuel refers to its resistance to the degradation processes that can change its fuel properties and e c make it inapplicable as a fuel 2, 3. A fuel is considered unstable when it undergoes changes, such n a as oxidation or autoxidation in the presence of oxygen in ambient air, thermal or thermal-oxidative v d decomposition because of heat, hydrolysis when in contact with water or moisture in tanks and A fuel lines, microbial contamination from water droplets containing bacteria or fungi, or migration C S of dust particles into the fuel 4, 5. R The stability of biodiesel includes the aspects of oxidation, thermal, and storage stability. Oxidation stability is the tendency of fuels to react with oxygen at ambient temperature 6, 7. Biodiesel degradation prior to combustion in diesel engine is affected by different factors, such as nature of the original lipid feedstock, biodiesel production process, storage and handling RSC Advances Page 4 of 60 conditions, fuel additives and impurities, conditions within the fuel tank, and fuel distribution system 8-11. Thermal stability involves the measurement of the tendency of a fuel to produce asphaltenes when exposed to high temperature conditions; asphaltenes are tar-like resinous substances generated in the fuel, and these substances plug the fuel filters of engines when used as fuel 12, 13. The temperature has a significant effect on oxidative degradation because it enhances t p the rate of degradation. Unstable oxidation products can attack elastomers 14. The oxidation of i r c biodiesel prompts the development of hydroperoxides, which can assault elastomers or polymerize s u to form insoluble gums. Oxidation products, such as hydroperoxides and carboxylic acids, can n a function as plasticizers of elastomers15. M d Storage stability describes the general stability of the fuel under long-term storage. Oxidative e t degradation is perhaps one of the initial concerns of storage stability, but microbial growth and p e water contamination are definite issues of storage stability in the long run 16. c c Several previous studies have already investigated the oxidation, thermal, and storage stability of A biodiesel 17-19. Few review articles have analyzed different aspects of biodiesel stability together s e c with the effects of oxidation inhibitors on engine performance and emission1, 20-23. Several test n a methods have been devised to measure the stability of biodiesel; these methods involve the v d treatment of fatty oil or ester under elevated temperature, time, and oxygen exposure while A measuring one or more oxidation-sensitive properties, such as peroxide value, insolubles, C S evolution of volatile short chain fatty acids, or heat of reaction 20, 24 25. However, no simple stability R test or single stability parameter currently exists to sufficiently indicate all the stability features of biodiesel fuel. A single new test that can completely define biodiesel stability is highly improbable because different tests have various functions. The present paper attempts to review the work conducted on the oxidation, thermal, and storage stability of biodiesel, various test methods, and Page 5 of 60 RSC Advances improvement of biodiesel stability. This article focuses on a comprehensive study of three different aspects of biodiesel oxidation stability, the methods applied to improve it, effect of oxidation inhibitors (i.e., antioxidants) on stability, and influence of antioxidant-treated blend on diesel engine performance and emission characteristics. t p i r 2. Different aspects of biodiesel stability c s Biodiesel stability is affected by interactions with atmospheric oxygen, light, temperature, storage u n conditions, and factors causing sediment formation26. Biodiesel produced from vegetable oils and a M other feedstocks possess lower stability compared with petroleum-based diesel because of the d unsaturated fatty acid content, such as linoleic and linolenic acids, on the fatty acid profile of the e t p parent feedstock27. Biodiesel stability depends on different fatty acid compositions. Most plant- e c derived fatty oils contain poly-unsaturated fatty acids that are methylene-interrupted rather than c A conjugated. This structural property is essential to the understanding of the stability. Thus, the s instability of biodiesel can be divided into three aspects, namely, oxidative, thermal, and storage e c instability. The instability of biodiesel is dependent on the quantity and configuration of the n a olefinic unsaturation in the fatty acid chains. v d 2.1. Oxidation stability A The oxidation of fatty acid chain is a complex method because of its various applications 28. The C S oxidation of biodiesel is caused by unsaturation in fatty acid chain and existence of double bonds R in the fatty acid molecule, which exhibits high levels of reactivity with O , particularly when 2 exposed to air or water. Unsaturated fatty compounds are used to mitigate oxidation stability, because low amounts of more highly unsaturated fatty compounds have a disproportionately strong effect in reducing oxidation stability29. Hence, the oxidation mechanism can be explained by two RSC Advances Page 6 of 60 categories, namely, primary oxidation and secondary oxidation. Numerous researchers have investigated the chemistry of primary and secondary oxidation 2, 9, 10, 14, 30-48. Several studies have reported that vinyl polymerization involves higher molecular weight oligomers of fatty oils or ester formation 16, 34. Primary oxidation occurs through a set of reactions categorized as initiation, propagation, and termination 20. As shown in Fig. 1, the first set includes the elimination of t p hydrogen from a carbon atom to make a carbon free radical. If diatomic oxygen is present, the i r c consequent reaction to form a peroxy radical becomes enormously fast, even not allowing s u substantial alternatives for the carbon-based free radical 35, 36. Carbon free radicals are more active n a than peroxy free radicals. However, peroxy free radical is adequately reactive to fast abstract M hydrogen atom to form another carbon radical and hydroperoxide (ROOH). The newly formed d e free carbon radical can react with diatomic oxygen and continue the propagation cycle. t p e Initiation: RH+I·→ R· + IH c c A Propagation: R· +O → ROO· 2 s e ROO· + RH → ROOH + R· c n Termination: R· + R· → R-R a v d ROO· + ROO· → Stable products A C Fig: 1. Basic oxidation reaction S R During the induction period, the ROOH residue concentrations remains low until a certain time interval, and the oxidation stability of fatty acid or biodiesel can be determined under stress conditions 5. Page 7 of 60 RSC Advances For the whole oxidation system, the ROOH level increases very quickly until the initial period is reached 2. During the initial period, ROOH can directly or indirectly change the properties of fatty oils and biodiesels 32. The maximum level of ROOH forms at 300–400 meq O2/kg at any ROOH concentration profile peak, although the level of higher ROOH has been investigated 49. The fatty acid reacts with the molecular oxygen and produces unstable peroxide radical (ROO ), which t p i further reacts with the original substrate RH. The transfer of a hydrogen atom from fatty acids to r c a peroxide radical will result in the formation of a fatty acid hydroperoxide (ROOH). The radical s u chain reaction is shown in Eq. (1), in which the reaction with oxygen results in the formation of a n a new fatty acid radical (R ), because of the addition of fatty acid hydroperoxide (ROOH) and self- M sustaining chain reaction. d e t p → R·+ O2 ROO· e c c A → ROO· + RH ROOH + R· (1) s e c The termination step is achieved when two free radicals react and form stable products, as shown n a in the following equations. v d R·+R·→R–R (2) A C ROO·+ROO·→stable products (3) S R When an adequate concentration of radical species is available, the probability of two radicals colliding is very high 11. Peroxyl radicals (ROO ) can produce peroxyl-linked molecules(R–OO– R) and liberating oxygen as follows in reaction (4) ROO+ROO→R–OO–R+O (4) 2 RSC Advances Page 8 of 60 The ROOH concentration remains very low during the primary period of oxidation until a certain time interval, and this time period is often referred to as the induction period (IP), The presence of temperature and oxygen pressure during IP is identified by the comparative exposure to oxidation of TAG or alkyl ester, thereby signaling the onset of rapid oxidation; the ROOH level rapidly increases when the IP has elapsed 11. The hydroperoxide (ROOH) levels can either peak and then t p decrease or increase and plateau at a steady state as oxidation progresses. Although issues, such as i r c extent of earlier oxidation, temperature, oxygen availability, and incidence of metal catalysts, are s u likely involved in these phenomena, the explanations for the two different activities remain n a unclear. ROOH disintegration continues because of a peak in ROOH concentration. Insufficient M levels of oxygen can slow or even stop ROOH formation. Similarly, different factors, such as d e higher temperature and presence of hydroperoxide decomposing metal catalysts (e.g., copper and t p e iron), which increase the ROOH decomposition rate, can influence ROOH concentration. At 300– c c 400 meq O /kg, maximum ROOH levels typically form 2, 11 although higher ROOH levels have 2 A been observed 11 s e c Numerous secondary oxidation products, including short chain carboxylic acids, alcohols, high n a molecular weight oligomers, and aldehydes, form even at ambient temperature during the v d secondary oxidation stages, whereas hydroperoxides (ROOH) continue to decompose and interact A 50. The secondary products are produced in different ways. Several studies reported various C S secondary oxidation products observed from different experiments using biodiesel, such as 25 R different aldehyde compounds during vegetable oil oxidation, including hexenals, heptenals, propane, pentane, and 2,4-heptadienal. 24, 51. Polymeric species form with the involvement of fatty acid chains. Trimmers or tetramers are smaller than polymeric spices 11, although the open literature does not explain the reason for this difference. Polymer formation increases viscosity. Page 9 of 60 RSC Advances C–O–C and C–C linkages produce fatty acids, esters, and aliphatic alcohols. Hasenhuettle 11 explained the decomposition mechanism of hydroperoxides to shorter chain fatty acids, such as formic acid. Table 1. demonstrates the oxidation stability or IP and unsaturated fatty acid composition for methyl esters of distinctive oils. These data were collected from the literature, and the details of oxidation of a specific biodiesel (ethyl linolate ester) are illustrated in Fig 2. Step 1: t p Hydrogen abstraction from the allyl group. Step 2: Oxygen attack at either end of the radical center, i r c producing intermediate peroxy radicals. Step 3: Monohydroperoxide formation. Step 4: Partial s u decomposition of the initially formed monohydroperoxides into oxo-products and water. n a M d e t p e c c A s e c n a v d A C S R