Introduction

Gas turbines are established in power generation with application in on-site generation, distributed power systems, oil and gas operations, and industrial processes. The possibilities of added arrangements such as steam cycles for combined cycle power plants, heat recovery boilers for combined heat and power systems have made gas turbines indispensable for large scale power generation, district heating and mechanical drive applications. Apart from the simple cycle, gas turbines could employ advanced cycles such as recuperated, reheat and intercooled cycles as well as utilise steam or water injection to improve work output, cycle efficiency and performance or drive emissions to reliable technical limits. These engines can be applied for peak, base or intermediate loads, especially when operating as multiple units [Pilavachi, 2000; Najjar, 2001; Polullikkas, 2004; Polyzakis et al. 2008].

Their advantages over reciprocating engines include:

 Large amount of useful work from a relatively small size and weight engine

 Capability for fuel flexibility (gas and distillate oil)

 Compact size

 Relatively low capital and maintenance cost

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3.1.1 Fuels and Engine Performance

Fuels are required to meet the following requirements at all operating conditions: a) Ease of flow

b) Ease of ignition

c) Good combustion properties d) High calorific value

e) Minimal negating effects on combustion components and turbine parts f) Minimal corrosion impact on fuel systems

g) Good lubricating and conducting property for cooling requirement h) Safe to use

i) Sufficiently high combustion efficiency

This is because fuels are critical for reliable and efficient operation of gas turbines. They enable the expansion of the working fluid by allowing chemically stored energy to be released in the presence of heat. Depending on the quality, composition and properties of the fuel along with ambient inlet conditions such as pressure, and temperature, the performance and integrity of engines could be significantly affected while cycle efficiencies could improve or deteriorate. This could affect the engine’s durability, availability, maintainability and reliability.

The common properties of fuels that are important for gas turbines include density, viscosity, and calorific value. Other important properties includes: lubricity, which prevents wears on metal surfaces and leaks around seals; flash point, a key parameter for good ignition; pour point; cloud point etc, but these are outside the scope of this study.

Table 3.1 presents the typical biodiesel fuel properties and as stated by ASTM D6751-15 for biodiesels and ASTM D2880-14a for diesel fuels.

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Table 3.1: Liquid Fuel Properties & Specifications [Arbab et al. 2013; Atabani et al. 2012]

Fuel Properties Diesel Fuel

ASTM D2880 Biodiesel ASTM D6751 Typical Biodiesel4 Density at 15oC (kg/cm3) 876 880 837-930 Kinematic Viscosity at 40oC (cSt) 1.35-2.46 1.9-6.0 2.61-5.9 Calorific value (MJ/kg) 42-46 - 33-42.73 Flash point (oC) 38 100-1707 69-259

Water and sediment content (vol. %) 0.05 0.056 <0.005-0.05 [0.02-4507]

Sulphur content (m/m %) 0.05 0.056 <0.005-0.02 [0.2-4747]

Lubricity (HFRR, µm) - - 135-280

1. Viscosity can be classified into dynamic and kinematic viscosity. The dynamic

viscosity refers to the resistance of the fuel to move over another fluid or surface and the kinematic viscosity refers to the ratio of viscous forces to inertia [Soares, 2008]. The dynamic viscosity is most applicable to liquid fuels performance because it determines the ability of a fuel to meet pumping requirement while kinematic viscosity determines the bulk conditions. According to Soares, [2008], liquids are not pumpable with kinematic viscosity of less than 1 cSt and atomization would be unsatisfactory for fuels with kinematic viscosity of less than 10 cSt. From Table 3.1, it can be observed that diesel fuels for gas turbine application are required to have a viscosity not more than 2.4 cSt but not less than 1.3 cSt, but typical biodiesel fuel exceeds this limit. Tate et al. [2006] observed that the kinematic viscosity of three biodiesels from soy, canola and fish oil were significantly higher than that of diesel fuel and decreased with temperature. This supports the general notion that biodiesels are more viscous than conventional diesel fuels, although some biodiesel fuels are in close range with diesel fuels as shown in Table 3.1.

Viscosity directly affects fuel flow rates, spray characteristics and atomizing properties of a fuel [Arbab et al. 2013]. A highly viscous fuel reduces evaporation rate, induces poor fuel atomization, and also increases the specific fuel consumption of a fuel pump.

4

Biodiesels from Jatropha, Palm, Coconut, Cotton seed, Sunflower, Safflower, Soybean, Canola/Rapeseed 5 Minimum 6 Maximum 7 ppm

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2. Density refers to the weight of a unit volume of fuel [Demirbas, 2008]. It is

expressed as specific gravity; that is, the density of the fuel to that of water at a defined temperature. Density is very closely related to viscosity and it increases the energy concentration of a fuel [Arbab et al. 2013]. From Table 3.1, it can be observed that biodiesel fuels have a wide range of density between 830 kg/m3 and 930 kg/m3. The density of diesel is typically in the range of 820 kg/m3 and 880 kg/m3 [Soares, 2008]. A high dense fuel would have relatively high viscosity and this would bring about poor combustion performance and emission characteristics.

3. Fuel Calorific Value can be expressed as net, the lower heating value (LHV) or

gross, the higher heating value (HHV). Unlike the HHV that incorporates latent heat of vaporization of the water generated with the combustion products, the LHV gives the net energy content. LHV is heat released under pressure in a constant volume, when the combustion products are cooled to the initial temperature of 25°C [Walsh and Fletcher, 2008]. In essence, it is the quantity of heat release during combustion. A high calorific value fuel improves combustion performance and vice versa. Usually, diesel fuels have LHV in the range of 42-46 MJ/kg, but biodiesels have much lower energy content in range of 33-42 MJ/kg while natural gas has LHV of about 47 MJ/kg [Soares, 2008]. There are concerns with the use of bio-fuels in engines because of the above described differences in fuel properties that is, relatively high viscosity, low volatility and low fuel calorific value. Properties such as viscosity and volatility induce smoking by affecting spray penetration, fuel mean droplet size and evaporation rates, which initiate local fuel rich spots [Lefebvre and Ballal, 2010]. And in order to improve such properties, crude bio-oil is often converted to biodiesel via transesterification. This form of conversion of bio-oil is said to reduce the viscosity of biodiesels by a factor of 8, molecular weight by a third while increasing volatility substantially [Gupta et al. 2010]. Rehman et al. [2011] also report a reduction in the viscosity of Jatropha biodiesel from 0.92 to 0.88, due to transesterification of crude bio-oil. Other means of reducing the viscosity of biodiesels significantly include heating, blending, dilution and emulsification [Rehman et al. 2011 and Arbab et al. 2013].

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Furthermore, changing one fuel property could significantly affect another [Lefebvre et al. 1985]. Demirbas [2008] observed that the various properties of fuels are closely related. An increase in the density of a biodiesel fuel from 0.85 to 0.89 kg/L resulted in a linear increase in viscosity from 2.83 to 5.12 mm2/s. Also, their heating value directly correlated with the physical properties of the biodiesel fuel. And, the failure of a fuel to meet fuel specification could negatively impact engine performance, emissions, engine materials and component life [Tan et al. 2013]. For instance, a decrease in specific gravity of fuel could result in less fuel flow pressure, necessitate the control system to cause a compensating volume of fuel to be released and this may lead to excessive temperature or over speeding of the engine [Soares, 2008; Lefebvre and Ballal, 2010]. There could be increase in soot formation, consequently increase in radiation and flame temperature. This increases the cooling requirement and reduces durability of rotating components. The next section describes how the properties of microalgae and Jatropha biodiesel fuels were integrated into the engine performance model for fuel analysis.

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3.2 Methodology