Influence of pyrolysis condition on switchgrass bio-oil yield and physicochemical properties

Abstract

The poor and inconsistent physicochemical properties of bio-oil are inhibiting its industrialized production. We investigated the variability in properties of switchgrass bio-oil produced at three pyrolysis temperatures ( T = 450, 500, and 550 °C) and three feedstock moisture contents (MC = 5%, 10%, and 15%) in a 3 × 3 factorial experiment in order to exploit opportunities to improve bio-oil properties through optimization of pyrolysis parameters. Results showed that even with the single type of feedstock and pyrolysis system, the two main factors and their interaction caused large variations in bio-oil yield and most of the measured physicochemical properties. Following improvements of bio-oil properties could be individually achieved by selecting an optimal pyrolysis condition (shown in parenthesis) comparing with the worst case: increase of bio-oil yield by more than twofold (MC = 10%, T = 450 °C), increase of pH by 20.4% from 2.74 to 3.3 (MC = 10%, T = 550 °C), increase of higher heating value by 18.1% from 16.6 to 19.6 MJ/kg (MC = 10%, T = 450 °C), decrease of density by 5.9% from 1.18 to 1.11 g/cm 3 (MC = 5%, T = 550 °C), decrease of water content by 36% from 31.4 to 20.1 wt.% (MC = 5%, T = 450 °C), decrease of viscosity by 40% from 28.2 to 17 centistokes (MC = 5%, T = 550 °C), decrease of solid content by 57% from 2.86 to 1.23 wt.% (MC = 15%, T = 550 °C), and decrease of ash content by 41.9% from 0.62 to 0.36 wt.% (MC = 15%, T = 550 °C). There is no single, clear-cut optimal condition that can satisfy the criteria for a bio-oil product with all the desired properties. Trade-offs should be balanced according to the usage of the end-products.