Pyrolysis of spirulina and zeolite cracking over HZSM-5. An analytical investigation on the chemical route of bio-oil from cultivation to combustion

•Aromatic bio-oil isolated from ZSM-5 catalytic pyrolysis of spirulina.•Had a high C content, fully deoxygenated, partially denitrogenated.•Burnt homogeneously with a sooting flame then heterogeneously as cenosphere.•Biochar/coke retained 60% carbon, fixed N can replenish almost half initial nutrient.•20% proposed as the maximum energy recovery to obtain a petrol-like algal biofuel.

Spirulina (Arthrospira platensis) was cultivated in a 70 L indoor vertical photobioreactor and harvested at concentrations of 1.0 g L−1 dry biomass. Lyophilised algal biomass was pyrolysed at 500 °C under nitrogen and vapours were passed over pelletised HZSM5- zeolite (SiO2/Al2O3 38). An organic fraction (bio-oil) overlaying an aqueous phase was obtained by cold trapping, while non-condensed bio-oil components (XAD fraction) were adsorbed onto a poly(styrene-co-divinylbenzene) resin. About 20% of the original algal carbon was converted into inorganic carbon in the aqueous (HCO3−/CO32−) and gas phase (composed of 70% CO2, 20% CO). Most of spirulina carbon ended up in char (30%) and coke (30%). Bio-oil and XAD fraction represented approximately 10% mass, 20% carbon and 20% energy of algal biomass. Bio-oil composition was dominated by alkylated monoaromatic hydrocarbons, with benzene concentrations below 10 g kg−1. Large part of original nitrogen was dissolved in the aqueous phase (40%) and incorporated into char/coke (37%). A minor fraction (6%) of nitrogen ended up in bio-oil in the form of indoles, pyrroles, carbazoles, anilines. While deoxygenation was effective, denitrogenation was incomplete and probably counteracted by zeolite ammonisation. Microcombustion experiments showed that the bio-oil burnt efficiently, but with a sooting flame, and a tendency to form small solid carbonaceous residues probably associated with the presence of heavy compounds.