Fast pyrolysis bio-oil oil is a promising alternative to fossil fuels and is currently entering the heating oil market. However, there is a lack of available information about the phase stability of bio-oil. The water-soluble and water-insoluble compounds in bio-oil can either be in one homogeneous phase or form two individual phases, to which we refer to as phase separation. Phase separation can occur immediately after condensation of the pyrolysis vapors to bio-oil because of certain pyrolysis conditions or type of raw material or after years of aging because of changes in composition caused by repolymerization reactions. We present how the phase separation of bio-oils is related to the chemical composition and show that the probability of phase separation can be predicted with a numerical stability index based on the chemical composition. The chemical composition of the bio-oils studied was characterized using a solvent extraction scheme that describes the composition of bio-oil as a blend of three macro fractions: C 1 −C 6 oxygenated molecules (named co-solvents), water-insoluble molecules, and watersoluble polar molecules (including water but excluding the co-solvents), e.g., anhydrosugars. The results show that the required amount of co-solvent to dissolve both fractions and keep the bio-oil homogeneous varies depending upon the chemical composition. The minimum amount of co-solvent for homogeneous bio-oils was observed to be from 15 to 30 wt %. The correlation between the chemical composition and homogeneity of fresh and aged bio-oils is shown in ternary-phase diagrams. Addition experiments were made with model compounds to cover a larger part of the phase diagram.

Fast pyrolysis bio-oil has unfavorable properties that restrict its use in many applications. Among the main issues are high acidity, instability, and water and oxygen content, which give rise to corrosiveness, polymerization during storage, and a low heating value. Esterification and azeotropic water removal can improve all of these properties. In this work, low acidity biooils were produced from fast pyrolysis bio-oil via esterification with methanol or n-butanol. Esterification conversion was enhanced by azeotropic water removal prior to and/or during esterification. An additional hydrocarbon entrainer (n-heptane or petroleum ether) was required for efficient water removal. The product oils had total acid numbers ranging from 5 to 10 mg KOH/g and pH values from 4.0 to 5.6. The best results were obtained with 1:0.9:0.1 wt ratio of bio-oil, n-butanol, and n-heptane and p-toluenesulfonic acid (p-TSA) as catalyst. Removal of homogeneous catalyst (2 wt % p-toluenesulfonic acid (p-TSA)) was attempted by precipitation, centrifugation, and water washing, but only 41−82 wt % of the catalyst could be recovered from the product oil based on sulfur content. Solid acid catalysts were more efficient with methanol than n-butanol in dry conditions. An organic base (triethylamine) was tested for neutralizing the methanol esterified bio-oil's residual acidity. Nitrogen content increased by 0.1−0.4 wt % when pH values of 6−8 were obtained.

The changes in chemical composition and physical properties that accompany bio-oil aging reactions have been studied earlier. However, one fundamental aspect of this transformation process has been ignored. In this article, we prove that aging of fast-pyrolysis bio-oils from woody biomass is an exothermic process with notable heat generation under adiabatic conditions. The heat generation characteristics of several fast-pyrolysis bio-oils were studied in a novel reaction calorimeter that was made in-house. When typical fast-pyrolysis bio-oils were stored at 50 °C for a period of 1 week, they exhibited overall adiabatic temperature increases ranging from 14 K to 28 K. The largest differences in heat generation were observed at the beginning of the aging period, which corresponds with the previously known reactivity characteristics of bio-oils. Increasing the storage temperature accelerated the aging reactions, which manifested as higher overall temperature increasesup to 55 K in 1 weekand higher specific thermal power density (STPD) values. The reactivity of the bio-oil at 70 °C could be partly passivated by employing a 1 week pretreatment at a more moderate temperature (40 °C). The addition of alcohol decreased heat generation from the bio-oil. The observed heat generation of bio-oils under varying aging conditions correlated with changes in their chemical composition and physical properties. This shows that previously developed bio-oil stability indicators can also be used to estimate the heat generation potential of a given bio-oil. In particular, a change in the concentration of carbonyl compounds exhibited a clearly linear correlation with heat generation. A decrease of one unit in the carbonyl content (mol/kg of bio-oil) would correspond to an adiabatic temperature increase of 20 °C.

No abstract