Co-Pyrolysis of Biosolids with Lignocellulosic Biomass and Alum Sludge: Effect of Feedstock on Product Yields and Properties

Abstract

Co-pyrolysis is a simple and effective method to reduce heavy metal concentration in biochar produced from biosolids. In the current study, the effect of co-pyrolysis feedstock on product yield and product properties were studied by mixing biosolids with alum sludge, wheat straw and canola straw in a 3:1 mass ratio and performing co-pyrolysis at 700°C  in a fluid bed reactor. It was hypothesized that synergistic effects in co-pyrolysis process would vary based on the feedstock. The results indicated that the addition of all three feedstocks considered in this study notably reduced As, Cd, Cr, Cu, Ni, Pb, Se and Zn concentrations in the biochar. However, Cu and Zn concentrations still exceeded the upper limit for unrestricted land applications according to EPA Victoria regulations. Alum sludge co-pyrolysis reported the lowest Cu and Zn concentrations at 412.99 mg/kg, and 453.31 mg/kg, respectively, due to higher biochar yieldr in alum sludge, which dilutes heavy metal present in resulting biochar co-pyrolysis compared to wheat straw and canola straw. Lignocellulosic biomass addition reduced biochar yield and improved C, H, and N content, HHV and thermal stability of biochar. Oil produced from alum sludge co-pyrolysis had a lower mass yield (27.0%) and fuel acidity (9.4%) with a higher proportion of nitrogenated compounds (20.69%) compared to wheat straw and canola straw. Furthermore, presence of synergistic effects were evident in the product yield distribution results of co-pyrolysis in all three feedstocks. However, the results also indicated that mechanisms of synergistic effects may depend on the feedstock type and composition.

Keywords: Biosolids, Alum sludge, Lignocellulosic biomass, Co-pyrolysis, Biochar, Heavy metals