Biomass pyrolysis polygeneration with bio-oil recycling: Co-pyrolysis of heavy bio-oil and pine wood leached with light bio-oil for product upgradation

[Display omitted] •Co-pyrolysis polygeneration of heavy bio-oil and pine sawdust is studied.•Light bio-oil is used to leach pine sawdust to remove ash and alkali metals.•Leaching pretreatment of pine sawdust improves the mass yield and quality of bio-oil.•Co-pyrolysis of pine sawdust and heavy bio-oil has obvious synergistic effect.•The sum of energy yield of biochar and bio-oil is more than 80%. The liquid products obtained from pyrolysis of biomass include light bio-oil and heavy bio-oil. The former contains large amounts of water and organic acids, while the latter contains more of pyrolysis lignin and phenolic compounds. Herein, a new method of pine wood co-pyrolysis polygeneration along with bio-oil recycling was proposed for upgradation of pyrolysis products. Light bio-oil was used for leaching of pine sawdust, instead of the traditional acid leaching pretreatment, followed by its co-pyrolysis with heavy bio-oil. The synergistic effect during co-pyrolysis polygeneration was studied. Results showed that leaching pretreatment could effectively remove the alkali metals (95.6 % of K) from pine wood, which improved the quality and energy yield of bio-oil during the subsequent pyrolysis. The maximum increase in mass yield and energy yield of bio-oil were 6.7 % and 7.1 %, respectively. The co-pyrolysis of pine wood and heavy bio-oil had obvious synergistic effect, which increased the mass yield of biochar (maximum increase: 3.3 %) and pyrolysis gas, while reducing the water and relative acids contents of bio-oil and improving the contents of CO and C2+ gases in the gaseous products. The maximum differences between the experimental and theoretical HHVs of biochar and bio-oil were 1.01 MJ/kg and 1.94 MJ/kg, respectively. Thus, this method provides a potential way to classify and utilize light and heavy components of bio-oil and also provides a basis for biomass pyrolysis polygeneration technology.