Research on the thermochemical conversion utilization of nitrogen-rich microalgae: Two-step catalytic pyrolysis of Nannochloropsis sp over ZSM-5

• Two-step catalytic pyrolysis characteristic of microalgae over ZSM-5 was studied. • High selectivities of N-heterocyclic compounds and hydrocarbons were achieved. • Two-step catalytic pyrolysis reaction pathways of microalgae was presented. To explore an effective method of utilizing nitrogen-rich microalgae, the two-step pyrolysis (TSP) characteristic of Nannochloropsis sp (NCS) over ZSM-5 were studied by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetric analyzer (TG). The results showed that the TG process of NCS was classified into three stages. ZSM-5 increased the final weight loss and the pyrolysis temperature of NCS. The kinetic analysis was conducted by Flynn-Wall-Ozawa method, and showed that the pyrolysis of NCS conformed to one-dimensional shrinking core model. ZSM-5 did not affect the kinetic model, but decreased the activation energy. Compared with one-step pyrolysis, TSP improved the selectivity of N-heterocyclic compounds in the first step. Catalytic TSP further reduced the complexity of products significantly, and increased the selectivity of olefins in the first step, nitriles and aromatic hydrocarbons in the second step. The possible catalytic TSP reaction pathways of NCS were proposed.