Viscosity of a bio-oil is the measure of its internal friction which resists the flow of it. Viscosity is an important fuel property that should be considered when attempting to design and select handling, processing and transportation equipment. Viscosity of bio-oil affects the operation of fuel injection equipment, particularly when the increase in the viscosity affects the fluidity of fuel at low temperatures. Again, the quality and practical application of bio-oil as fuel is closely dependent on its viscosity and the elemental compositions i. e., the lower viscosity and oxygen content is desirable (Ertas & Alma, 2010). In general, bio-oil has high viscosity as compared to crude oil and diesel fuel (Onay & Kockar, 2006; Parihar et al., 2007; Pootakham & Kumar, 2010a, b). Pootakham and Kumar (2010a) reported that the loading equipment of the petroleum product such as gasoline and diesel fuels operates between 0.9 and 1.3 m3/ min, whereas they can be operated for bio-oil at a volume flow rate of 0.6 m3/min and an operating pressure of 205 kPa (or 30 psi) for safety (Jones & Pujado, 2006). Bio-oil is more viscous than crude oil at room temperature; however its viscosity is very similar to that of crude oil in a temperature range of 35-45°C, (Bridgewater, 1999; Thamburaj, 2000; Pootakham & Kumar, 2010a, b). In order to transport the bio-oil in pipeline, the temperature of the pipeline should be maintained in the range of 35-45°C to keep the viscosity similar to that of crude oil (Pootakham & Kumar, 2010a, b). According to Thangalazhy-Gopakumar et al (2010), viscosity of bio-oil is relatively higher than that of diesel (0.011 Pa. s) and gasoline (0.006 Pa. s). In general, high viscosity fuel results in poor atomization and incomplete combustion, formation of excessive carbon deposits on the injection nozzles and the combustion chamber, and contamination of the lubricating oil with unburnt residues. The viscosity of the fuel directly influences atomization and mixing in the combustion chamber. In fuel application, the lower the viscosity, the easier it is to pump and to atomize and achieve finer droplets (Ji-Lu, 2008). Hence bio-oils in their original form are not suitable for use in modern diesel engines (Ozaktas et al., 1997). Because of their high acidity, low thermal stability, low calorific value, high viscosity, and poor lubrication characteristics limit their use as transportation fuel (Garcia-Perez et al., 2006b). Oasmaa et al (2005) stated that for engine application, the viscosity should be in the range of 10-20 cSt with a solids content of less than 0.1 wt%. As is known, bio-oils are entirely different from petroleum fuels. There is a necessity to establish fuel specifications for commercial application of bio-oils as liquid fuels. The specifications should include the most critical properties such as viscosity, lubricity, homogeneity, stability, heating value, pH, water, flash point, solids, and ash (Qiang et al., 2008).

The viscosity of bio-oil varies depending on the temperature, feedstock, water content of the oil, amount of light ends that have been collected and the extent to which the pyrolysis oil has aged (Ji-Lu, 2008). For example, bio-oil produced from P. indicus and F. mandshurica had a kinetic viscosity of 70-350 mPa s and 10-70 mPa s separately, and bio-oil produced from rice straw had a minimum kinetic viscosity about 5-10 mPa s, which is mainly due to high water content in bio-oil from rice straw (Luo et al., 2004). The presence of water has both negative and positive effects on the storage and utilization of bio-oils. The negative effects are, it lowers heating values, causes phase separation, increases ignition delay, and reduces combustion rates and adiabatic flame temperatures during the combustion process. Further, it leads to premature evaporation and subsequent injection difficulties during the preheating process. The positive effects are, it reduces viscosity, facilitates atomization, and reduces pollutant emissions during combustion (Calabria et al., 2007). Moreover, OH radicals from water can inhibit the formation of soot and can also accelerate its oxidation. According to Senso’z and Kaynar (2006), viscosity of the bio-oils is related to fatty acid chain length and number of saturated bonds. In general, the density of bio-oil is higher than that of water confirms that it contains heavy fractions (Sensoz et al., 2006). The lignin content of original feedstock has a positive influence on molecular weight and viscosity of bio-oil (Fahmi et al., 2008). Recently, Ertas and Alma (2010) compared the average molecular weight and molecular weight distribution of laurel extraction residues bio-oil (664 g mol/l and 1.52) and found they were very close to those of switchgrass bio-oil of 658 g mol/l and

1. 49, respectively (He et al., 2009a). Viscosity of bio — oil increases during storage, due to slow polymerization and condensation reactions, the increase in viscosity is enhanced by higher temperature. The presence of inhibitors (hydroquinone) can dramatically reduce the rate of increase in bio-oil viscosity, due to the suppression of thermal polymerization reactions by the inhibitors (Ji-Lu, 2008). Garcia-Perez et al (2010) observed that the increase in viscosity of bio-oils is due to the solubilization of lignin derived oligomers. The condensation reactions occur ageing increases the water content in bio-oil with time (Garcia-Perez et al., 2002). The instability may be attributed to the presence of alkali metals in the ash, which are being carried over/entrained by the char particles with the vapors. These alkali metals catalyse the polymerization reactions and thereby increase the viscosity (Diebold, 2002).

Simple methods such as addition of polar solvents, diesel or other fuels can address some of the undesired bio-oil characteristics. Polar solvents, such as methanol or ethanol, can improve the volatility and heating value and decrease the viscosity and acidity. The addition of ethanol improves the volatility, stability and heating value and decreases the viscosity, acidity and corrosivity (Ji-Lu & Yong-Ping, 2010). The blending of diesel or other fuels can rearrange the viscosity of bio-oil (Onay & Kockar, 2006). In order to improve fuel properties of bio-oils, many methods are under investigation such as emulsification, hydrotreating, and catalytic cracking, which is beyond the scope of this chapter.

2. Materials and methods