Coupling effects of heating pelleting and torrefaction on black pellets production from microalga Nannochloropsis Oceanica residues

•Heating pelleting of MNOR improve physical, transportation and thermal characteristics.•Torrefied MNOR pellets can be upgraded as lignite.•Two kinds of acid with high-value are detected from tar.•The mechanism of MNOR torrefaction is revealed by the two-step kinetic model. Microalga Mannochloropsis Oceanica residues (MNOR) are a potential bioenergy source that can become fossil fuel substitute. Its low density, high moisture and content of carbon and oxygen content hinder its wide application. Heating pelleting coupled with torrefaction is an effective approach to update MNOR. In this study, the effects of compression time, compression load and mold temperature were investigated in self-made heating pelleting device. The cost-effective combination for pelleting were 30 s, 1.0 MPa and 75 °C, where the pellets had high shatter resistance and relaxation density. The results of torrefaction experiments in tubular furnace showed that CO2 was main gas product released from MNOR pellet. Two kinds of acid with high-value were detected from tar by GCMS. FTIR and XPS analyses confirmed that the torrefaction enhanced the carbonation of the MNOR pellets, where the relative content of C- (C, H) increased and the relative content of C = O(–OH) decreased. The two-step first-order kinetic model was used to reveal the mechanisms of MNOR torrefaction, with activation energies ranging from 4.98 to 74.95 kJ/mol for powder and from 5.83 to 77.71 kJ/mol for pellet.