Optimal Co-pyrolysis of municipal sewage sludge and microalgae Chlorella Vulgaris: Products characterization, synergistic effects, mechanism, and reaction pathways

Co-pyrolysis of municipal sewage sludge (MSS) and microalgae Chlorella Vulgaris (MCV) was carried out in a fixed-bed reactor to investigate the effect of feedstocks interaction on pyrolysis products and synergistic effects. Based on previous authors' research, the optimum values of reaction parameters were T = 520 °C, mixing ratio (MCV/MSS) = 0.82, and an inert gas flow rate of 0.55 L/min. For the investigation of synergetic effects at optimum point, theoretical values anticipated from each characteristic were computed as an arithmetic sum of the single pyrolysis values obtained. For the first time, by using GC analysis, changes in gas components (H2, CO, CO2, CH4) at different temperatures (220–520 °C) and reaction times (0–30 min) have been investigated. Referring to GC-MS results the percentages of oxygen-containing species, alkanes, alkenes, aliphatic, aromatic, and total hydrocarbons were changed from the theoretical value of (41.719%), (6.004%), (11.215%), (17.219%), (4.317%), and (21.536%) to experimental value (39.292%), (3.693%), (12.103%), (15.796%), (0%), and (15.796%) respectively. Fatty acids and N&O-containing compounds (amides) were increased from (12.297%) to (26.009%) and (6.989%) to (16.217%), respectively. Co-pyrolysis bio-oil was rich in fatty acids and amides compounds. So, co-pyrolysis of sewage sludge and Chlorella Vulgaris provides a new orientation for high value-added utilization of biomass. It was observed that the nitrogen and oxygen content of biochar was reduced (0.29% and 1.01% respectively) compared to the theoretical condition. In the end, a comprehensive mechanism, synergistic, and reaction pathways of co-pyrolysis of MCV and MSS were presented. [Display omitted] •Co-pyrolysis favored gaseous production and the inhibition of other products.•Prolonged reaction time favorable for non-condensable gaseous products.•Nitrogenous chemicals in bio-oil mainly were fatty amides.•The possible mechanism and reaction pathways were given.