Mild Pyrolysis of Manually Pressed and Liquid Nitrogen Treated De‐Lipid Cake of Nannochloropsis Oculata for Bioenergy Utilisation

Abstract

AbstractDue to the damaging impacts of continued use of fossil fuels, there is global interest in developing sustainable biofuel production to reduce society's dependency on carbon based energy resources. Microalgae cultivation can contribute to CO2 fixation from the atmosphere, while simultaneously producing a source of lipids from the biomass for third generation biodiesel fuel production. The residual de‐lipid cake left after lipid extraction can be treated with thermochemical techniques (such as mild pyrolysis) to produce solid biochar as an end product with a higher energy density and lower moisture content offering advantages for downstream processing or carbon sequestration. De‐lipid cake was produced by solvent extraction from Nannochloropsis oculata that had been manually pressed and/or treated with liquid nitrogen (LN2). The de‐lipid cake was thermally treated at 200 °C or 300 °C under partial vacuum in an oxygen free atmosphere. The solid biochar produced had a reduced moisture content (MC) resulting in a mass reduction of 25 and 66 wt % of de‐lipid cake without LN2 and treatment at 200 °C and 300 °C, respectively, while with LN2 treated cake the mass reduction was 23 and 67 wt % at 200 °C and 300 °C. The higher heating value of the control sample (without any manual pressing or LN2 treatment) was 23.35 MJ kg−1, while for the control sample it was enhanced to 26.82 and 30.56 MJ kg−1 with treatment at 200 °C and 300 °C, respectively. With LN2 treated samples with pressing the HHV was 21.98 MJ kg−1 for control sample as compared to 25.90 and 28.72 MJ kg−1 at 200 and 300 °C respectively, where the lower values were observed because of the lipid removal. The measured gas pressure developed, likely due to the production of CO2 and CH4 as major gases,, was 0.19 and 0.53 bars without LN2 treatment samples, while it was 0.13 and 0.58 bars with LN2. The torrefaction process (mild pyrolysis) energy analysis showed that the ER (energy ratio) without LN2 treatment sample with 0.485 at 200 °C was the highest and the lowest 0.407 energy ratio was found with LN2 treated sample at the higher treatment temperature (300 °C).