1.2 Basic Elemental Conversions in Biomass Processing

In order to be used for production of biofuels and chemicals, biomass needs to be depolymerized and deoxygenated. Deoxygenation is required because the presence of O in biofuels reduces the heat content of molecules and usually gives them highpolarity, which hinders blending with existing fossil fuels (Lange, 2007). Chemical applications may require muchless deoxygenation, since the presence of O often provides valuable physical and chem­ical properties to the product. Biomass feedstocks usually have an amount of carbon which must be retained throughout the value chain, few hydrogen, which must be added, and too much oxygen, which must be rejected along with other undesirable elements (such as nitro­gen and sulfur). Hydrogen is usually added as water (H2O), even if this implicates an addition of extra oxygen, which must be rejected. The addition of hydrogen as H2 is more attractive and efficient (using proper metal catalysts) but underprivileged by the fact that elemental hydrogen is not present in nature and energy must be invested to produce it. Oxygen is rejected either as CO2 or H2O. In both cases, there are elemental issues: in the first case every mole of oxygen removes half a mole of carbon (thus reducing carbon efficiency), while in the second case 1 mol of oxygen removes 2 moles of hydrogen (which, contrarily, needs to be added). It would be most desirable to reject oxygen as O2, but this is not a typical output of any biomass conversion process. The other undesired elements, sulfur and nitrogen, are usually rejected in their oxide forms (SO2 and NO2, respectively), thus contributing to rejec­tion of excess oxygen.