Bio-oil derived from fast pyrolysis of lignocellulosic materials is among the most complex and inexpensive raw oils that can be produced today. Although commercial or demonstration scale fast pyrolysis units can readily produce this oil, this industry has not grown to significant commercial impact due to the lack of bio-oil market pull. This paper is a review of the challenges and opportunities for bio-oil upgrading and refining. Pyrolysis oil consists of six major fractions. (water 15-30 wt.%, light oxygenates, 8-26 wt. %, mono-phenols, 2-7 wt.%, water insoluble oligomers derived from lignin 15-25wt.%, and water soluble havey molecules 10-30 wt.%). The composition of water soluble oligomers is relatively poorly studied. In the 1880s bio-oil refining (formally known as wood distillation) targeted the separation and commercialization of C1-C4 light oxygenated compounds to produce methanol, acetic acid and acetone with the commercialization of the lignin derived water insoluble fraction for preserving purification techniques. Strategies for biofuels production are discussed in section four. Bio-oil derived products are discussed in the last section. 2. Bio-oil Composition The study of bio-oil chemical composition has been the subject of active research in the last twenty years 2,24-35. Pyrolysis oil contains numerous oxygenated compounds, which include carboxylic acids, water, alcohols, esthers, anhydrosugars, furanics, phenolics, aldehydes, and ketones covering a wide range of molecular weights and functionalities 2,24,29,30,36-41. The specific composition is directly related to the feedstock and the conditions used in their production 42-44. Water is typically quantified by Karl Fischer titration 2 and is the most abundant bio-oil compound accounting between 15 and 30 wt. % 2 (See Figure 1). Water forms mostly from dehydration reactions of carbohydrate depolymerized products in the liquid intermediate 44. Gas Chromatography/Mass Spectroscopy (GC/MS) is by far the most common technique for the quantification of the pyrolysis oil organic volatile fraction 2,24,27,45. GC/MS detectable compounds typically account for between 30 and 40 wt. % 2. Table 2 shows the range of compounds quantified by GC/MS reported in the literature. Only four molecules (glycoaldehyde, acetic acid, acetol and levoglucosan) are found in quantities sufficiently high (>5 wt. %) to justify their separation and commercialization as chemicals. Methanol can also be produced in quantities justifying its commercialization but hardwood has to be used as feedstock. The remainder of the oil if refined is likely to be commercialized as fractions (mono-phenols, pyrolytic lignin, anhydrosugars, pyrolytic humins, and hybrid oligomers). Because bio-oil consists of hundreds of compounds with concentrations below 0.5 wt. % it is desirable to express their chemical composition in terms of few chemical groups or families 24. This idea was first proposed by Hallet and Clark 46. The authors 46 modeled bio-oil evaporation rates using a model based on this characte...
Thin films (∼115 μm thick) of milled wood lignin from hybrid poplar and acid-washed hybrid poplar were pyrolyzed at 500 °C and ∼55 °C/s at five pressures (4, 250, 500, 750, and 1000 mbar) to determine the impact of secondary liquid intermediate reactions on the product distribution. For both milled wood lignin extracted from poplar and acid-washed hybrid poplar wood, pressure had a significant effect on the product distribution for thin film pyrolysis between 4 and 1000 mbar. For lignin, lowering the pressure from 1000 mbar to 4 mbar reduced the char yield from 36 to 23% and enhanced production of large cluster pyrolytic lignin. However, the pressure did not dramatically impact the gas yield (CO 2 , CO, methane, H 2 , ethane, propane, and butane), nor did it significantly impact the release of monomeric phenolic compounds. ICR-MS shows limited changes in the composition of lignin oligomers. The increase in the production of large lignin oligomers observed by UV fluorescence and the reduction of char yield with vacuum confirm the importance of oligomeric combination reactions to form large polyaromatic structures in the liquid intermediate. For hybrid poplar, lowering the pressure from 1000 mbar to 4 mbar decreased the char yield from 19 to 7% and enhanced production of heavy sugars (cellobiosan and cellotriosan). ICR-MS results clearly show the importance of dehydration reactions in the liquid intermediate. Lowering the pressure also enhanced production of CO, CO 2 , and methane due to heterogeneous catalysis by residual alkali and alkaline earth metals in the solid wood matrix. However, it also decreased production of levoglucosan from 10 to 6.1 wt %. The yields of levoglucosan and cellobiosan obtained for hybrid poplar were higher and lower, respectively, compared with those expected if the pyrolysis products were the result of the additive contribution of hybrid poplar constituents. This result could be explained by the tendency of lignin liquid intermediate to bubble vigorously, contributing in this way to the removal of cellulose oligomers from the liquid intermediate.
A new methodology has been proposed to describe of the dynamics of bubble formation during pyrolysis of Organosolv lignin and sucrose (surrogates for biomass) using fast speed visualization (125fps) with mathematical modeling. The model uses a population balance to predict overall rates of bubble birth and death, bubble bursting, and aerosol ejection. The experimental studies were performed on a uniquely modified CDS Analytical Pyroprobe 5000 to visualize the formation of bubbles within the liquid intermediate phase at heating rates close 100 o C/s. Experimentally, we observed that bubbles follow a log-normal distribution versus bubble size within the liquid intermediate phase for both materials. This distribution function changes over time due to increased viscosity from solidification reactions that generate char and the changes in the rate of bubble formation. Micro-explosion intensity was used to estimate aerosol ejection intensity. The model predicts aerosol ejection yields of 21.18% w / w from Organosolv lignin and 17.40% w / w from sucrose during pyrolysis with an average droplet size of 10 m.
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