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Energies 2019, 12, 809 25 of 41 Green diesel has a higher cetane number (above 70) than petroleum diesel or biodiesel. Cetane number (CN) is a measure of the ignition quality of a diesel fuel in a reciprocating CI internal combustion engine. It is measured according to the ASTM D613 standard and quantifies the ignition delay which is the time interval from the injection of the fuel to the beginning of the combustion process (see Section 7.2). The higher the cetane number of a fuel the shorter is the ignition delay and usually this means that the engine may operate with a higher thermal efficiency and lower fuel consumption. Fuels such as green diesel which contain high concentrations of normal (straight chain) saturated hydrocarbons have high cetane numbers because the activation energy required for the formation of free radicals to begin the combustion process is low compared to branched hydrocarbons or aromatics which are more stable and need higher pressures and temperatures to autoignite. When green diesel is mixed to petroleum diesel the cetane number increases almost linearly to the blending ratio and the automotive manufacturers claim that such a CN enhancement is preferable to the utilization of CN improvement additives [216]. As discussed previously, the normal saturated hydrocarbons that provide green diesel its high CN rating also cause the cloud point of the fuel to be above 20 ◦C. The cloud point is measured experimentally according to the ASTM D2500 standard and signifies the highest temperature below which the formation of solid wax particulates make the fuel hazy. The high cloud point makes the liquid impractical for use as an engine fuel or blendstock and isomerization is required which reduces both the cloud point and the CN rating. Isomerization has the advantage that it does not change the density of the fuel. Instead, the regulation of the low temperature operability of the fossil petroleum is made through distillation which provides lighter hydrocarbons. This lowers the density and the viscosity of the fuel and increases fuel consumption. Finally, it is important to notice that green diesel is much more adjustable for low temperature operation than biodiesel in which the cold properties depend on the feedstock. Smagala et al. [217] examined the effect of the isomerization on the CN number and the cloud point and shown that an initial mixture of normal saturation hydrocarbons with a CN rating of about 118 and a cloud point of 20 ◦C can be converted into a practical green diesel fuel of a cloud point down to −30 ◦C while keeping the CN number above 70. Another property which indicates the low temperature operability of the diesel fuel is the Cold Filter Plugging Point (CFPP). This is measured by the constant cooling of a sample at a rate of 40 ◦C/h and signifies the highest temperature at which 20 ml of the fuel fail to pass through a 45-micron wire mesh under a pressure of 2 kPa in less than 60 s. In petroleum diesel the CFPP may be improved by means of Cold Flow Improver (CFI) additives. However, due to the narrow distillation range of the green diesel hydrocarbons between C15 and C18 the effect of these additives in the green diesel is small. Neste [216] claims that the CFPP is usually equal or marginally lower than the cloud point of the fuel and that the low temperature operability is controlled mainly by adjusting the cloud point. Green diesel has a lower density than biodiesel or petroleum diesel [108]. This means that the volumetric heating value of the fuel is reduced and the engine fuel consumption increases. Luckily, the lower heating value (LHV) of green diesel on mass basis is higher than petroleum diesel and this counteracts the adverse effect of the lower density to some extent. This is due to the higher H2 content of green diesel. The lower density may also offer to some extend financial benefits since it can allow the blending with heavier petroleum products of lower cost. Lubricity is extremely important in a CI engine since both the fuel pumps and the fuel injectors are high precision equipment manufactured with the highest possible geometrical accuracy and operating with highly loaded contacts [218,219]. These are essential to the accurate quantification of the fuel injection and play a key role to the optimization of the engine performance for high efficiency and low fuel consumption. Consequently, the high fuel lubricity is particularly important for the unproblematic engine operation with minimal wear [219,220]. The lubricity of a diesel fuel can be estimated experimentally through the High Frequency Reciprocating Ring (HFRR) method according to the ASTM D6079 standard. A hardened steel ball oscillates across a hardened steel plate underPDF Image | Green Diesel: Biomass Feedstocks, Production Technologies
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