Empirical Approach for Predicting the Cetane Number of Biodiesel

EMPIRICAL APPROACH FOR PREDICTING THE CETANE NUMBER OF BIODIESEL

P. K. BOSE *

Mechanical Engineering, Jadavpur University, Kolkata 700032, India

(Received 5 July 2007; Revised 26 July 2008)

ABSTRACT−The cetane number is an indicator of ignition quality and thus of fuel quality in the realm of diesel engines. It is conceptually similar to the octane number used for gasoline. Generally, a compound that has a high octane number tends to have a low cetane number and vice versa. The cetane number of a diesel fuel is related to the ignition delay time. In our work the first approach is a statistical one the accuracy of which depends upon the data obtained from various papers and literature sources, as all equations used were based on this data. During prediction using more than one equation is a good approach, as it provides the accuracy as well as the relative error. The second approach is also a statistical one, but its value depends upon the saponification number and iodine value. Therefore the accuracy of this equation may be higher, since we can collect the data for saponification numbers and iodine values from literature, without needing to calculate them. Using the saponification number and iodine value we can select an optimal biodiesel as generally a good biodiesel is selected using these three values. Thus the second approach allows us the freedom to select a biodiesel.

KEY WORDS : Cetane number, Biodiesel, Saponification value, Iodine value

1. INTRODUCTION of biodiesel fuels by means of additives known as cetane improvers. Despite the inherently relatively high cetane

A long straight-chain hydrocarbon, hexadecane (C16H34; numbers of fatty compounds, NOx exhaust emissions trivial name of cetane, giving the cetane scale its name) is usually increase slightly when operating a diesel engine on the high quality standard on the cetane scale with an assign- biodiesel. The connection between the structure of fatty ed cetane number of 100. A highly branched compound esters and exhaust emissions is investigated by studying the

2,2,4,4,6,8,8-heptamethylnonane (HMN, also C16H34), a exhaust emissions caused by using enriched fatty acid alkyl compound with poor ignition quality, is the low-quality esters as fuel. NOx exhaust emissions reportedly increase standard and with an assigned cetane number of 15. The with increasing unsaturation and decreasing chain length, two reference compounds on the cetane scale show that the which can also lead to a connection with the carbon cetane number decreases with decreasing chain length and numbers of these compounds. Particulate emissions, on the increasing branching. Aromatic compounds, which occur other hand, are hardly influenced by the aforementioned in significant amounts in conventional diesel fuel (DF), structural factors. The relationship between the carbon especially DF2, have low cetane numbers but their cetane number and engine emissions is complicated by many number increases with increasing size of n-alkyl side factors including the technology level of the engine. chains (Knothe and Dunn, 2001). The cetane scale is In this paper we have concentrated in two methods. arbitrary and compounds with CNN100 or CNb15 have been identified. The standard ASTM D975 for conven- PREDICTING THE CETANE NUMBER OF tional DF requires a minimum cetane number of 40 while ALCOHOLS AND METHYL ESTERS FROM the standards for biodiesel prescribe a minimum of 47 THEIR PHYSICAL PROPERTIES (ASTM D6751) or 51 (EN14214). N-Hexadecane Due to the high cetane numbers of many fatty compounds, which can exceed the cetane scale, the term “lipid combustion quality number” is suggested for these compounds (Katwal and Soni, 2003). For conventional diesel Hexadecane (cetane), cetane Number=100 fuel, higher cetane numbers have been correlated with reduced nitrogen oxides (NOx) in exhaust emissions. This CETANE NUMBER CALCULATION FOR BOTH correlation has led to efforts to improve the cetane number UNSATURATED AND SATURATED METHYL ESTERS USING THEIR IODINE VALUE AND *Corresponding author. e-mail: [email protected] SAPONIFICATION NUMBER

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