Didem Ozgimen and Sevil Yucel

Yildiz Technical University, Bioengineering Department, Istanbul

Turkey

1. Introduction

The depletion of fossil fuels and their effects on environmental pollution necessitate the usage of alternative renewable energy sources in recent years. In this context, biodiesel is an important one of the alternative renewable energy sources which has been mostly used nowadays. Biodiesel is a renewable and energy-efficient fuel that is non-toxic, biodegradable in water and has lesser exhaust emissions. It can also reduce greenhouse gas effect and does not contribute to global warming due to lesser emissions. Because it does not contain carcinogens and its sulphur content is also lower than the mineral diesel (Sharma & Singh, 2009; Suppalakpanya et al., 2010). Biodiesel can be used, storaged safely and easily as a fuel besides its environmental benefits. Also it is cheaper than the fossil fuels which affect the environment in a negative way. It requires no engine conversion or fuel system modification to run biodiesel on conventional diesel engines.

Today, biodiesel is commonly produced in many countries of the world such as Malaysia, Germany, USA, France, Italy and also in Australia, Brazil, and Argentina. Biodiesel production of EU in 2009 was presented in Table 1 (European Biodiesel Board, July 2010). As can be seen from Table 1, 9 million tons biodiesel were produced in European Union countries in 2009. Germany and France are the leaders in biodiesel production. EU represents about 65% of worldwide biodiesel output. Biodiesel is also main biofuel produced and marketed in Europe. In 2009, biodiesel represented is about 75% of biofuels produced in Europe.

The world production of biodiesel between 1991 and 2009 was presented in Figure 1. From Figure 1, biodiesel production increased sharply after 2000s in the world.

Firstly in 1900, Rudolph Diesel showed that diesel engines could work with peanut oil. And then, the different kinds of methods such as pyrolysis, catalytic cracking, blending and microemulsification were used to produce biodiesel from vegetable oil for diesel engines (Sharma & Singh, 2009; Varma & Madras, 2007). Finally, transesterification process was developed as the most suitable method to overcome problems due to direct use of oil in diesel engines (Varma & Madras, 2007).

Biodiesel is generally produced from different sources such as plant oils: soybean oil (Kaieda et al., 1999; Samukawa et al., 2000; Silva et al., 2010; Cao et al., 2005; Lee et al., 2009; Yu et al., 2010), cottonseed oil (Kose et al., 2002; He et al., 2007; Royon et al., 2007; Hoda, 2010; Azcan & Danisman, 2007; Rashid et al., 2009), canola oil (Dube et al., 2007; Issariyakul et al., 2008), sunflower oil (Madras et al., 2004), linseed oil (Kaieda et al., 1999), olive oil (Lee et al., 2009), peanut seed oil (Kaya et al., 2009), tobacco oil (Veljkovic et al., 2006), palm oil (Melero et al., 2009), recycled cooking oils (Issariyakul et al., 2008; Rahmanlar, 2010; Zhang et al. 2003; Demirba§, 2009) and animal fats (Da Cunha et al., 2009; Oner & Altun, 2009; Guru et al., 2009; Guru et al., 2010; Tashtoush et al., 2004; Teixeira et al., 2009; Chung et al., 2009).

The major economic factor to consider for input costs of biodiesel production is the feedstock. 90 % of the total cost of the biodiesel production is the resource of the feedstock. Studies to solve this economic problem especially focused on biodiesel production from cheaper raw material. Using agricultural wastes, high acid oils, soapstock, waste frying oil and alg oil as raw materials for biodiesel production are being reported in literature (Haas & Scott, 1996;Ozgul & Turkay, 1993; Ozgul & Turkay, 2002; Leung & Guo, 2006; Yucel et al., 2010; Ozgimen & Yucel, 2010).

Table 1. Biodiesel production of EU in 2009 (EBB 2010)

Transesterification process, as showed in Figure 2 (Barnard et al., 2007) is a conventional and the most common method for biodiesel production. In transesterification reaction homogeneous catalysts (alkali or acid) or heterogeneous catalysts can be used. The catalysts split the oil into glycerin and biodiesel and they could make production easier and faster.

(FAME)

Fig. 2. Biodiesel production via transesterification reaction (Barnard et al., 2007)

In this method, fatty acid alkyl esters are produced by the reaction of triglycerides with an alcohol, especially ethanol or methanol, in the presence of alkali, acid or enzyme catalyst etc. The sodium hydroxide or potassium hydroxide, which is dissolved in alcohol, is generally used as catalyst in transesterification reaction (Dube et al., 2007). The products of the reaction are fatty acid methyl esters (FAMEs), which is the biodiesel, and glycerin (Vicente et al., 2004). Ethanol can be also used as alcohol instead of methanol. If ethanol is used, fatty acid ethyl ester (FAEE) is produced as product (Hanh et al., 2009b). Methyl ester rather than ethyl ester production was preferred, because methyl esters are the predominant product of commerce, and methanol is considerably cheaper than ethanol (Zhou & Boocock, 2003). However, methanol usage has an important disadvantage, it is petroleum based produced. Whereas ethanol can be produced from agricultural renewable resources, thereby attaining total independence from petroleum-based alcohols (Saifuddin & Chua, 2004; Encinar et al. 2007). Ethanol is also preferred mostly in ethanol producing countries. Propanol and butanol have been also used as alcohols in biodiesel production.

Alkali-catalyzed transesterification proceeds much time faster than that catalyzed by an acid and it is the one most used commercially (Dube et al., 2007; Freedman et al., 1984). The most commonly used alkali catalysts are NaOH, CHeONa, and KOH (Vicente et al., 2004). Potassium hydroxide (KOH) and sodium hydroxide (NaOH) flakes are inexpensive, easy to handle in transportation and storage, and are preferred by small producers. Alkyl oxide solutions of sodium methoxide or potassium methoxide in methanol, which are now commercially available, are the preferred catalysts for large continuous-flow production processes (Singh et al., 2006).

For acid-catalyzed systems, sulfuric acid has been the most investigated catalyst, but other acids, such as HCl, BF3, H3PO4, and organic sulfonic acids, have also been used by different researchers (Lotero et al, 2005). But in alkali catalyzed method, glycerides and alcohol must be substantially anhydrous, otherwise it leads to saponification (Helwani et al., 2009). Due to saponification the catalytic efficiency decreases, the separation of glycerol becomes difficult and it also causes gel formation (Helwani et al., 2009). In homogeneous catalyzed reactions, separation of catalyst from the reaction mixture is hard and expensive. With this purpose, large amount of water is used to separate catalyst and product (Vyas et al., 2010). On the other hand, undesired by-product formation such as glycerin can be seen, the reaction lasts very long and energy consumption may be very high. Thus, researchers have focused on development of new biodiesel production methods and the optimization of the processes (Sharma et al., 2008). So, various processes such as supercritical process,
microwave assisted method and ultrasound assisted method have recently developed. Alternative energy stimulants or non-classical energies have been used for many years to increase the reaction rate and to enhance the yield of particular reaction products. Novel methods or combining innovative methods and techniques are a challenge that can lead to unexpected advances in biodiesel production techniques (Nuechter et al., 2000). In this study, biodiesel production in supercritical conditions, in microwave and ultrasound techniques as novel methods through the years (2000-2011) was reviewed and presented in detail.