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].

Diesel

range

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