(117a) Refinery Feedstock From Partially Hydrogenated Biomass Pyrolysis Oil

Biomass provides a local, relatively secure energy source and may be produced with low or even negative carbon emissions.  Therefore biofuels are desirable replacements for petroleum-derived motor-fuels.  One method for making biofuels is fast pyrolysis of biomass followed by liquid separation to produce pyrolytic lignin.  This might then be hydrotreated to produce a liquid suitable for mixing into a refinery, where it would be further converted into a finished motor-fuel, taking advantage of the existing infrastructure and economies of scale of refineries. The organic compounds in the aqueous by-products could be reformed to produce the hydrogen needed to hydrotreat the lignin.

Pyrolytic lignin contains about half of the energy of the original biomass but consumes less hydrogen than whole pyrolysis oil when hydrogenated because it has much less oxygen.  It is much less corrosive than the whole oil because of the removal of carboxylic acids, and is potentially more stable because of the extraction of unstable oxygenates. 

To test this hypothesis, several pyrolytic lignins were prepared.   Whole pyrolysis oil was mixed with water at 1:1 and 3:1 water-to-oil ratios.  The carboxylic acidity in the pyrolytic lignin was reduced to 24 and 10 mgKOH/g-lignin compared to the 81 measured in the whole oil. The carbon content increased from the whole oil to 58-59% on a wet basis in the pyrolytic lignins.  These pyrolytic lignins were hydrotreated with hydrotreating catalysts made of sulfided nickel-molybdenum/alumina, platinum/char, or palladium/activated-carbon in a semi-batch 1 L stirred autoclave. The oil was stabilized under hydrogen at 150-280°C, then water and light organics flashed off by partial depressurization.  Then hydrodeoxygenation was performed at 340-400°C.  The total system pressure was controlled at 70 or 170 bar by addition of hydrogen gas.  Gas composition was monitored and condensable vapors were collected throughout experiment.  The organic fractions of the condensed vapors and the organic residue remaining in the autoclave were analyzed separately.  The residue constituted 65-85% of the organic product.

At hydrodeoxygenation temperatures of 355-400°C under hydrogen at 170 bar total pressure 52–78% of the carbon in the pyrolytic lignin was converted to organic liquid. The organic oxygen in the overall product, consisting of the organic condensates and the organic residue, was low (2-4%).  The carboxylic acidity was reduced to 1-7. Volatility by proximate analysis was higher for the organic condensates (97-99%), than the organic residues (93-99%), but was still high overall (93-99%).  Most of the products were miscible in heptanes and toluene at 10:1 .   However, insufficient miscibility of some liquids, especially the condensate collected during stabilization, reduced the yield of product deemed satisfactory.  Nevertheless with all three catalysts over 60% of the carbon in the pyrolytic lignin was converted to liquids with satisfactory properties.  At the less severe condition (T = 340°C, p = 70 bar), the products were not satisfactory. 

Analysis of the products by NMR and GC-MS showed that most of the product is aliphatic but the remaining oxygen is associated with the aromatic materials.  The products have a high content of cyclic aromatics and aliphatics with one or two rings.

Thus, the reforming of the aqueous liquids to produce hydrogen coupled with the hydrotreating of pyrolytic lignin is a technically feasible process way to convert biomass to hydrocarbon liquids that are promising refinery feedstocks.