Study of the Effect of Storage Time on the Oxidation and Thermal Stability of Various Biodiesels and Their Blends A. M. Ashraful, H. H. Masjuki, M. A. Kalam, S. M. Ashrafur Rahman,* M. Habibullah, and M. Syazwan Centre for Energy Sciences, Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia ABSTRACT: Biodiesel can be described as a safe alternative fuel, which can replace petroleum diesel in the future. It consists of long-chain fatty acid methyl esters (FAME). Biodiesel has high lubricity and is a clean burning fuel. It also produces less air pollution, is renewable biodegradable, and is safer for the environment. Since biodiesel is produced from vegetable oil, animal fats, used cooking oil, and so forth, which may contain more or less unsaturated fatty acids that are prone to oxidation accelerated by exposure to air during storage and at high temperature, it may yield polymerized compounds. The oxidation and thermal stability of the fuel changes with storage time due to the formation of oxidation. Therefore, the aim of this study to evaluate the stabilities of biodiesel according to measured fuel properties, such as density, viscosity, flash point, total acid number (TAN), and total base number (TBN), by using various methodologies. In addition, oxidation stability of the samples was measured by the induction period using a Rancimat instrument. In this experiment, palm oil methyl ester (PME), palm biodiesel blend (40% PME and 60% diesel fuel), jatropha methyl ester (JME), jatropha biodiesel blend (40% JME and 60% diesel fuel), coconut oil methyl ester (COME), and conventional diesel fuel were used. Experiments were carried out at intervals over a 12-week test period. The experimental results for JME and PME showed similar performance in terms of flash point. All samples met the standard specification of the American Society for Testing and Materials (ASTM) D6751 (3 h) regarding the induction period, except for JME and its biodiesel blend, which did not meet the EN 14214 (6 h) standard specification. Among the fuel samples giving the worst results for TBN value due to oxidation, overall, among the biodiesels, PME and COME were found to give better results with respect to oxidation and storage stabilities. 1. INTRODUCTION Biodiesel is an alternative fuel source which is produced by using simple chemical processes on waste vegetable oils or fat oils. It can be used in a diesel engine without needing any engine modification. It is also known as a green fuel because the advantages include renewability and the reduction of most regulated exhaust emissions (it does not contribute to carbon dioxide (CO 2 ) emissions). 1 Biodiesel is safer for both the air and water. In its pure form, it is nontoxic and biodegradable, which is especially important in sensitive or protected waterway areas. It is also free from sulfur and aromatics, which reduces harmful emissions. When added to petroleum diesel, it makes fuel burn more cleanly. 2 However, biodiesel has the prominent technical problems of oxidation and thermal and storage instability. 3 Biodiesel is produced using the transesterification process. This process involves a reaction between triglycerides with an alcohol in the presence of base-catalyzed. 4 Short-chain alcohols, such as methanol and ethanol, can be used in the transesterification process. Based on lower-cost and faster- reacting characteristics, methanol is typically preferred. Alkyl ester and glycerol are the primary products of the reaction. Oxidative stability is defined as the ability of biodiesel to resist oxidization when exposed to factors such as air, water, and certain metals. Normally during long-term storage, biodiesel is more sensitive to oxidation than petroleum derivate diesel. The oxidation stability of biodiesel normally depends on the fatty acid profile of the parent feedstock. Thus, biodiesel that consists of high concentrations of unsaturated fatty acids, such as linoleic and linolenic, will tend to oxidize. 5 Oxidation stability is an important parameter that generally describes the degradation of biodiesel and is quite familiar in the context of problems with engine parts. 6 Peroxides and hydroperoxides are the main products of the oxidation process. The products produced from degradation normally have shorter-chain compounds, such as low molecular weight acids, aldehydes, ketones, and alcohols. 7 The presence of alcohols and acids will decrease the flash point and increase total acidity and the risk of corrosion. In addition, high molecular weight materials are formed through reactions of unstable hydroperoxide species with another fatty acid chain. Thermal stability can be described as the ability of biodiesel to resist breakdown or change in the chemical structure if exposed to heat over a long period of time. The overall stability of biodiesel will be reduced when it is exposed to UV irradiation, high temperature, and the presence of metal traces (contaminants). Therefore, it will affect quality and, hence, marketability. During oxidative degradation, biodiesel parame- ters, such as kinematic viscosity, cetane number, and acid value, are affected. 8 Temperature had a significant effect during oxidation degradation. When biodiesel is exposed to high temperature conditions, thermal stability involves the measure- ment of the tendency of a fuel to produce asphaltenes. 9 Received: October 6, 2013 Revised: January 15, 2014 Article pubs.acs.org/EF © XXXX American Chemical Society A dx.doi.org/10.1021/ef402411v | Energy Fuels XXXX, XXX, XXX-XXX