Torrefaction and process energy budget analysis of powdered, de-oiled, and in situ transesterified flaxseed cakes for energy generation

Edible vegetable oil seeds are used to produce oil for human consumption and inedible vegetable oil seeds are used to produce biofuels; the residue cakes from the extraction process contain significant energy that can be utilised. This study is a continuation of previous work on oil extraction from flaxseed to produce biodiesel; herein, the residue cake from flaxseed was studied. Three different types of samples (powdered seeds, de-oiled cake, and in situ transesterified cake of flaxseed) were torrefied (through slow pyrolysis), and the sample properties were characterized. The higher heating value (HHV=36.01±0.36 MJ kg−1) and bulk density (0.33±0.01 g cm−3) of the powdered seeds are higher compared to the other two samples (i.e. de-oiled cake and in situ transesterified cake). The torrefied biomass was light weight losing its mechanical strength on treatment, making it easier to grind. It was observed that the biomass torrefied at 300 °C had a higher fraction of particles between sieve sizes (between 2.0 and 0.063 mm) compared to the biomass torrefied at 200 °C. The sieve sizes between 0.5 and 0.063 mm showed that more particles of the torrefied biomass treated at 300 °C were collected compared to that torrefied at 200 °C. Due to the loss of hydrogen and oxygen content and the removal of water, the carbon content in the torrefied biomass increased by 25.52, 24.51, and 22.69 % for the powdered seeds, de-oiled cake, and in situ cake samples, respectively. The powdered seeds had a higher carbon and hydrogen content compared to the de-oiled and in situ transesterified flaxseed cakes because the powdered seeds contained fatty acids. The powdered seeds had a higher concentration of CO2 gas liberated during treatment compared to the de-oiled and in situ transesterified cakes. This was attributed to the powdered seeds having a higher concentration of volatile hydrocarbons and a lower ash content. It was observed that the energy ratios were reduced with respect to increasing temperature whereas the highest energy ratios were found for the powdered seeds