Thermochemical conversion of biomass to biofuels is a highly accelerated form of the geologic processes that created petroleum fossil fuels. This mode of conversion consists of passing biomass through a heated reactor in the absence of oxygen at or above ambient pressures. Residence times within the reactor are dictated by the system type and vary from tenths of a second to up to an hour (NSF 2008). Within the reactor, most of the biomass is pyrolyzed into small molecules that flow out of the reactor as gasses. Depending on the severity of the reactor conditions, the major products are liquids or gasses, plus non-fuel by-products such as tars and mineral-rich char.

When the reactor temperatures are low (~100-750°C), most of the pyrolysis products condense when cooled to produce a liquid referred to as bio-oil. Bio-oil is a complex hydrocarbon mixture that includes water (~25%); 1- to 4-carbon alcohols, acids, and aldehydes (total ~45%); carbohydrates (~10%); and phenolics and other lignin derivatives (~20%) (NSF 2008). Additional heating in the presence of chemical catalysts upgrades and distills these products to form higher chain-length, less oxygenated hydrocarbons. Attractively, these upgraded mixtures are suitable for use in conventional combustion engines with or without blending with petroleum-derived fuels.

At higher reactor temperatures (750-1200°C) and in the presence of some oxygen, the most useful pyrolysis products are CO and H2, which are referred to as syngas. CO2, H2O, H2S, and other impurities also form (NSF

2008) . Of course, H2 in itself is a high-energy fuel molecule, though use as a transportation fuel is not yet technically feasible. For immediate needs, after a cleaning step, the CO and H2 can be recombined with heating and catalysts to form alkanes, via Fischer-Tropsch synthesis, or alcohols, especially

methanol. Another method of upgrading syngas to transportations fuel is known as indirect fermentation. In this process, anaerobic bacteria can utilize the CO to form ethanol, and in limited cases, butanol (Mohammadi et al. 2011). Bacteria can also couple the oxidation of CO to CO2 to produce H2 (Oelgeschlager et al. 2008).

Relative to current biochemical conversion, the short residence times within the reactor bed provide the possibility for distributed production of thermo-converted fuels, reducing the distance that low density biomass must be transported for biofuel production (NSF 2008). On the other hand, a major challenge for thermochemical processes is optimizing the energy efficiency and the fraction of carbon from the biomass that is incorporated into the final, useable fuel. A wide diversity of feedstocks, including switchgrass (Boateng et al. 2006), can be used for thermochemical conversion, and this technology has been seen as having the advantage of being largely feedstock-independent. However, scientists have recently begun to explore possible correlations between feedstock content and syngas and bio-oil formation (Boateng et al. 2006; Gan et al. 2012). Desirable biomass qualities for thermochemical biofuel production may rely on the details of the method of conversion. These qualities include higher content of reduced compounds (i. e., lignin) to maximize the starting material energy potential, lower nitrogen and mineral content to prevent these molecules from catalyzing oil degradation, and reduction of crosslinks between biomass components to allow staged conversion for the different biomass content fractions.