Biomethane and biocrude oil production from protein extracted residual Spirulina platensis

•Protein extracted residual Spirulina platensis biomass was evaluated as a biofuel feedstock.•Yield and rate of methane production were higher compared to original S. platensis.•Yield and nitrogen content of biocrude were lower compared to original S. platensis.•Performance of protein extracted biomass was better in AD (anaerobic digestion) than HTL (hydrothermal liquefaction).

The performance of the residual biomass obtained after protein extraction from the cyanobacterium Spirulina platensis as a feedstock for biomethane production via AD (anaerobic digestion), and biocrude oil production via HTL (hydrothermal liquefaction) pathways was investigated. The experimental methane yield and kinetic rate of methane production from PERB (protein extracted residual biomass) were higher by 30.4% and 161% respectively, compared to ORIB (original S. platensis biomass). The rate of methane production for PERB was also higher (by 38.9%) than high pressure homogenizer DISB (disrupted biomass), although the yield was slightly lower (by 7.8%). The lag phase time for methane production was the least for PERB among all the three substrates. On the other hand, HTL of PERB resulted in biocrude oil with slightly lower nitrogen content than ORIB (6.2% and 7% respectively), although at a reduced yield. A composition analysis using GC–MS revealed that the biocrude from PERB had a higher number of hydrocarbons and fatty acids and lower number of nitrogenous compounds compared to that from ORIB. A comparison of energy output and energy recovery in the AD and HTL processes suggested a better performance of PERB in the former process.