Microalgae for biofuel: Isothermal pyrolysis of a fresh and a marine microalga with mass and energy assessment

Abstract

Two microalgae species were pyrolysed to produce bio-oil and biochar at various temperatures to investigate the trade-off between the energy used in the pyrolysis and the energy released upon combustion. Chlorella vulgaris (a freshwater microalga) and Nannochloropsis oculata (a marine microalga) were chosen as representatives of commonly used freshwater and marine microalgae species. The Chlorella used here was high-protein (49.2%), low lipid (5.9%) and low ash (5.2%) content, with a higher heating value (HHV) of 20.9 MJkg−1. The Nannochloropsis was lower in protein (26.8%) but higher in lipids (12.3%) and ash (15.9%), with a HHV of 15.5 MJkg−1. The highest energy efficiency was for pyrolysis of Nannochloropsis at 400 °C, in terms of both bio-oil (58.1%) and biochar (52.7%) production. The highest efficiencies of Chlorella vulgaris were 42.4% for bio-oil at 500 oC and 45.2% for biochar at 400 °C. This work shows that, beyond Nannochloropsis’ advantage of not requiring freshwater for cultivation, it is also more energy efficient.