Heteroatoms are atoms other than carbon (C) and hydrogen (H) and are often encountered into bio — and fossil — based feedstocks. They include sul­fur (S), nitrogen (N) and in the case of bio-based feedstocks oxygen (O). In particular oxygen removal is of outmost importance as the presence of oxygen reduces oxidation stability (due to carboxylic and carbonylic dou­
ble bonds), increases acidity and corrosivity (due to the presence of water) and even reduces the heating value of the final biofuels. The main deoxy­genation reactions that take place include deoxygenation, decarbonylation and decarboxylation presented in Schemes 7, 8 and 9 respectively [7]. The main products of deoxygenation reactions include n-paraffins, while H2O, CO2 and CO are also produced, but can be removed with the excess hy­drogen within the flash drums of the product separation section. It should be noted however that these particular reactions give the paraffinic nature of the produced biofuels, and for this reason the hydrotreated products are often referred to as paraffinic fuels (e. g. paraffinic jet, paraffinic diesel etc)

R-CHjCOOH + 3H2 ——- ► R-CH2CH3 + 2-H20

SCHEME 7

R-CH2COOH + H2 ——— ► R-CH3 + CO + H20

SCHEME 8

R-CH2COOH + H2 ——- ► R-CH3 + co2

SCHEME 9

The other heteroatoms, i. e. S and N are removed according to the clas­sic heteroatom removal mechanisms of the fossil fuels in the form of gas­eous H2S and NH3 respectively.