A comparison of conventional and oxidative torrefaction of microalga Nannochloropsis Oceanica through energy efficiency analysis and life cycle assessment

Torrefaction is a promising method for producing microalgal solid biofuel. This study compared conventional torrefaction and oxidative torrefaction for microalgal solid biofuel production. Particular attention was paid to evaluating energy efficiency analysis and life cycle assessment, and several parameters and indexes were adopted and calculated. The results showed that mild oxidative torrefaction was the most energy-efficient operation, with the maximum value of relative upgrading energy index and relative energy return on investment. The life cycle assessment indicates that oxidative torrefaction had a lower environmental impact than conventional torrefaction, mainly due to its lower carrier gas consumption and electricity input. The environmental impact positively correlated with the torrefaction severity. The obtained results in this study can provide new insights on improving the cognition of energy efficiency evaluation and environmental impact assessment of the torrefaction process, which helps select optimal torrefaction conditions. [Display omitted] •Oxidative torrefaction possesses a higher energy efficiency, especially for mild torrefied.•The eco-invent 3.4 database of openLCA software with the CML 2001 method is adopted.•Nine different indexes are adopted for evaluating the environmental impact of torrefaction.•Oxidative torrefaction possesses a lower environmental impact compared to conventional torrefaction.•Environmental impact of torrefaction is ranked as severe torrefaction > mild torrefaction > light torrefaction.