Even though liquid biomass is currently being exploited as a renewable feedstock for fuels production, its characteristics are far beyond suitable for its use as fuel. More specifically liquid biomass, just as other types of biomass, has a small H/C ratio and high oxygen content, lowering its heat­ing value and increasing CO and CO2 emissions during its combustion. Moreover liquid biomass contains water, which can cause corrosion in the downstream processing units if it’s not completely removed, or even in the engine parts where its final products are utilized. In addition to the above, liquid biomass has an increased concentration in oxygenated compounds, mainly acids, aldehydes, ketones etc, which not only reduce the heating value, but also decrease the oxidation stability and increase the acidity of the produced biofuels. For all the aforementioned reasons it is impera­tive that liquid biomass should be upgraded and specifically that its H/C should be increased while the water and oxygen removed.

The effectiveness of catalytic hydroprocessing towards improving these problematic characteristics of liquid biomass is presented in Table 1, where the H/C ratio, the oxygen content and density before and after cata­lytic hydrotreatment of basic liquid biomass types are given. The H/C ratio exhibits a significant increase that exceeds 50% in all cases. This is due to the substitution of the heteroatoms by hydrogen atoms as well as in the saturation of double bonds that enriches the H/C analogy. The oxygen con­tent (including the oxygen contained in the water) from over 15%wt can be decreased down to 5wppm. Actually the deep deoxygenation achieved via catalytic hydrotreatment is the most significant contribution of this biomass conversion technology, as it improves significantly the oxidation stability of the final biofuels. Furthermore significant improvement is also observed in the biomass density, which is never below 0.9 kg/l while after hydrotreatment it reduces to values less than 0.8 kg/l

Catalytic hydroprocessing has been proven as the most efficient tech­nology for the upgrading of liquid biomass as it achieves to increase the H/C ratio and to remove oxygen and water. However the effectiveness of this technology is also shown in other parameters. For example the distil­lation curve of raw liquid biomass shows that over 90% of its molecules have boiling points exceeding 600°C and only 5% are within diesel range (220-360°C), while after catalytic hydrotreatment upgrading most of 90% of the product molecules are within diesel range [13].

TABLE 2: Fatty acid composition of most common vegetable oils [14][15] C8:0 C10:0 C12:0 C14:0 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 C20:0/C22:0 C20:1/ C22:1 EU Rapeseed oil 0.0 0.0 0.0 0.0 3.5 1.0 1.5 12.5 15.0 7.5 9.0 50.0 Soybean oil 0.0 0.0 0.0 0.3 8.2 0.5 4.5 25.0 49.0 5.0 7.5 0.0 Sunflower oil 0.0 0.0 0.0 0.0 6.0 0.0 4.2 18.8 69.3 0.3 1.4 0.0 Corn oil 0.0 0.0 0.0 1.0 9.0 1.5 2.5 40.0 45.0 0.0 0.0 1.0 non-EU Palm oil 0.0 0.0 0.0 3.5 39.5 0.0 3.5 47.0 6.5 0.0 0.0 0.0 Peanut oil 0.0 0.0 0.0 0.5 8.0 1.5 3.5 51.5 27.5 0.0 7.5 0.0 Canola oil 0.0 0.0 0.1 0.1 4.7 0.1 1.6 65.9 21.2 5.2 1.2 0.0 Castor oil 0.0 0.1 0.2 10.6 1.4 9.5 29.7 29.7 41.3 3.3 3.8 0.0

In the following sections the basic types of liquid biomass and their corresponding products via catalytic hydrotreatment are presented.