Co-pyrolysis of microalgae and sewage sludge: Biocrude assessment and char yield prediction

High feedstock price is an important barrier for microalgae pyrolysis to alternative biofuels, while high ash content and low heat value affect the stable operation of sewage sludge pyrolysis reactors. Co-pyrolysis of microalgae and sewage sludge can avoid the drawbacks in individual sludge pyrolysis and improve pyrolysis performances. For better understanding co-pyrolysis kinetics, biocrude characteristics and interaction of sewage sludge and microalgae, thermogrametric analysis (TGA) and fixed pyrolysis bed experiment have been conducted and carbon distribution and components in biocrude were evaluated based on quality and quantity. For TGA, there was nearly no difference between individual pyrolysis and co-pyrolysis below 550 °C, while obvious interaction was found from 550 °C to 700 °C in co-pyrolysis. According to analysis of co-pyrolysis kinetics, solid-phase decomposition reaction mechanism in sewage sludge individual pyrolysis turned into random nucleation and subsequent growth mechanism above 550 °C. For co-pyrolysis, there was a yield increase of C4 and C7 and a yield decrease of C9 in carbon distribution. Detailed comparison of biocrude composition and carbon distribution indicated interaction in the product of co-pyrolysis. Excellent linear relationship between H/C of feedstocks and pyrolysis char was observed. Co-pyrolysis products reduced in hydrocarbons and N-containing compounds but increased in ketones and aldehyde. Co-pyrolysis with microalgae biomass was superior to pyrolysis individual with sewage sludge, which could improve stable operation of sewage sludge pyrolysis system due to higher heat value of microalgae addition. Co-pyrolysis of microalgae and sewage sludge is a promising way to decrease feedstock cost and realize alternative fuel production.