Two-step catalytic pyrolysis of microalgae combining in-bed CaO with vapour phase upgrading over Ni-ZSM-5/bentonite extrudates

Abstract

A novel two-step upgrading process was developed for efficient catalytic cracking, deoxygenation and denitrogenation of pyrolysis vapours derived from microalgae, accompanied by significant carbon and nitrogen fixation in the pyrolysis char. Microalgae (Nannochloropsis gaditana) were pre-mixed with an equal amount of CaO (NG–CaO) and then pyrolyzed at 500 °C. The volatiles were further upgraded ex situ through a bed of Ni-ZSM-5/bentonite extrudates (NG–CaO–Ni) at 500 °C in a fluidized bed of quartz sand equipped with a semi-continuous feeding system. A total feed of 80 g microalgae was achieved per experiment by feeding at a rate of ca. 100 g h⁻¹. The transformation of C-and N-containing compounds during catalytic pyrolysis of microalgae was thoroughly examined, by a complete evaluation of the element redistribution over the four product phases (aqueous phase, heavy phase, non-condensable gases and char). As a reactant, CaO caused the fixation of 41.1% mass fraction of feedstock-N in the char and the capture of CO₂ at 0.26 g g⁻¹ microalgae. As a catalyst, CaO facilitated reactions including dehydration, decarboxylation, ring-opening and cracking of N-heterocyclic compounds. In the two-step NG–CaO–Ni pyrolysis mode, the heavy phase liquid yield reached 20.6 wt% (on microalgae feed basis), with a carbon yield of 40.0 wt% (on microalgae feed carbon content basis). Regarding the elemental composition of the bio-oil's heavy phase (on dry basis), this process resulted in an 89.4% deoxygenation degree and a 38.9% denitrogenation degree. The two-step upgrading process enhanced the formation of aromatic hydrocarbons by pre-cracking the feedstock to form aromatic precursors in the presence of CaO. Cracking and Diels–Alder reactions were further intensified on the acid sites of Ni-ZSM-5/bentonite extrudates. CaO effectively controlled the transformation of nitrogen-containing compounds, reducing the release of harmful N-pollutants (e.g., NH₃, HCN) while enhancing carbon retention in char. Additionally, this process significantly promoted the production of valuable aromatic hydrocarbons (up to 3.8 wt% in the heavy phase), offering a sustainable route to convert N-rich biomass into high-value chemicals and stable carbon sequestration products. These results highlight the potential of this approach to address environmental challenges associated with nitrogen-rich waste, enabling both resource recovery and pollution mitigation.