Heat Generation during the Aging of Wood-Derived Fast-Pyrolysis Bio-oils

Abstract

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.

Alcohol Stabilization of Low Water Content Pyrolysis Oil during High Temperature Treatment

“…Figure shows viscosity measurements as a function of shear rate at 40 °C of oil/1-propanol, oil/2-propanol, and oil/1-octanol before and after heat treatment at 200 °C. Heat treated samples of oil/1-propanol and oil/2-propanol showed very similar behaviors; viscosity increased rapidly in the first 6 h and then leveled off over the next 17 h. These trends are consistent with those observed in aging below 90 °C ,, with the most rapid property changes occurring within the first days with significantly slower changes afterward. The viscosity of oil/1-octanol samples continued to increase with aging time over the duration of 50 h. This extended time may be due to the pyrolysis oil/1-octanol sample not having reached chemical equilibrium even after 1 week of storage at room temperature.…”

Historical Review on VTT Fast Pyrolysis Bio-oil Production and Upgrading

“…Aging reactions also cause heat generation, which has to be notified especially when planning commercial-scale storage. There is a clear correlation of the increase in the adiabatic temperature of FPBO to the decrease in carbonyl compounds of FPBO as a result of aging (Figure 6) 70 for various bio-oils, using various temperatures, alcohol additions, or acid catalysts.…”

“…The displayed trendline and correlation coefficient are for a linear regression model containing all data points. This figure was reproduced with permission from ref . Copyright 2016 American Chemical Society.…”

Review on Aging of Bio-Oil from Biomass Pyrolysis and Strategy to Slowing Aging

“…The aging of bio-oil is an exothermic process, and heat generation during aging is dependent on the temperature and bio-oil reactivity based on the aging experimental results of bio-oils in a reaction calorimeter. 125 Sundqvist et al 125 proved that a change in the concentration of carbonyl compounds linearly correlates with heat generation during bio-oil aging. Hence, heat generation during aging can also be suggested as an indicator of bio-oil stability.…”