A detailed product analysis of bio-oil from fast pyrolysis of demineralised and torrefied biomass

- Biomass properties were altered prior to pyrolysis by acid leaching and torrefaction pretreatments.
- Acid leaching reduced inorganic fraction in biomass.
- Torrefaction reduced the moisture and acetyl content of biomass.
- The pretreatments improved the stability and reduced the inorganic, organic acid, water, and aldehyde content of the bio-oil.

Acid leaching and torrefaction were investigated as biomass pretreatments for fast pyrolysis of Pinus radiata. Both pretreatments were required to limit competitive primary and secondary reactions of pyrolysis vapours catalysed by water, organic acids, and inorganics present in the biomass. Torrefaction reduces the cost of acid leaching by providing the leaching reagent and eliminating the need for biomass rinsing prior to the leaching. It also reduces the biomass grinding costs, which helps offset the additional process costs of pretreating biomass. Acid leaching was required because pyrolysis of solely torrefied biomass was constrained by the high torrefaction temperatures required for significant bio-oil improvements, leading to low bio-oil yields due to the mass loss during torrefaction and increased char formation during pyrolysis. Finally, the reduced thermal conductivity of dry torrefied biomass increased the time avaliable for inorganics to catalyse primary reaction pathways during the biomass heating. The optimal pretreatment conditions were 1% acetic acid leaching at 30 °C for 4 h followed by torrefaction at 270 °C for 20 min. Pyrolysis of raw biomass yielded (dry basis) 55.3 wt% bio-oil, 25.0 wt%, char, and 12.5 wt% non-condensable gas, whereas pyrolysis of pretreated biomass yielded 57.8 wt% bio-oil, 23.7 wt%, char, and 11.5 wt% non-condensable gas. Integrating leaching and torrefaction as biomass pretreatments significantly reduced the heterogeneous and homogeneous secondary reactions of pyrolysis vapours. This integrated pretreatment improved the bio-oil's quality in terms of the organic acid (2.46-0.16%), water (16.8-3.6 wt%), aldehyde (1.58-0.50%), high molecular weight compound (10.2-4.2%), and inorganic (0.162-0.091 wt%) content, as well as the stability.