Microwave catalytic co-pyrolysis of mulberry branches and Chlorella vulgaris over nickel-loaded phosphorus-modified biochar

Developing efficient and low-cost catalysts is crucial for improving microwave pyrolysis efficiency of biomass. In this work, biochar catalysts (Ni/BC, P1-Ni/BC, P2-Ni/BC, and P3-Ni/BC) were prepared by loading nickel and doping phosphorus, and the catalysts with different addition (5%, 10%, 15% and 20%) were used in the co-pyrolysis of Chlorella vulgaris (CV) and mulberry branch (MB). The pyrolysis characteristics, product yield, and bio-oil components of catalytic co-pyrolysis were analyzed. Furthermore, the effect of phosphorus addition on the catalytic performance of nickel-based biochar was investigated. The results showed that compared with no-catalysts group (CV:MB = 1:1, named as C1M1), four biochar catalysts at 15% addition could significantly optimize the pyrolysis characteristics, promote the generation of pyrolysis gas. The best catalytic effect of pyrolysis characteristics appeared in 15% P1–Ni/BC, with the maximum Rv (0.0202 wt%/s) and Mt (77.10 wt%). Furthermore, the highest bio-oil yield (25.55%) appeared in 15% P2–Ni/BC. Compared with C1M1, catalysts increased the phenols content and decreased the oxygen-containing compounds such as alcohols and esters. The catalytic effect of P2-Ni/BC was the most obvious, phenols increased by 13.52%, and oxygen-containing compounds (except phenols) decreased by 11.76%. The denitrification performance of the Ni/BC catalyst could be improved by doping phosphorus, P1-Ni/BC exhibited excellent denitrification performance of bio-oil with the highest bio-oil denitrification rate (6.62%). The activity of P2-Ni/BC was 0.73 times the initial activity after repeated use three times, and most Ni and P were leached after pyrolysis while remaining little in P2-Ni/BC. This work provides theoretical guidance for biomass co-pyrolysis under a new biochar catalyst, having practical significance for appreciating and utilizing biomass and waste resources, contributing to renewable energy development.