Influence of the Pyrolysis Temperature on Sewage Sludge Product Distribution, Bio-Oil, and Char Properties

Abstract

Fast pyrolysis may be used for sewage sludge treatment with the advantages of a significant reduction of solid waste volume and production of a bio-oil that can be used as fuel. A study of the influence of the reaction temperature on sewage sludge pyrolysis has been carried out using a pyrolysis centrifugel reactor (PCR) at 475, 525, 575, and 625 °C. Maxima of both organic oil yield of 41 wt % on a dry ash free feedstock basis (daf) and a sludge oil energy recovery of 50% were obtained at 575 °C. The water-insoluble fraction, molecular-weight distribution, higher heating value (HHV), and thermal behaviors of sludge oils were found to be considerably influenced by the applied pyrolysis temperatures. The sludge oil properties obtained at the optimal temperature of 575 °C were a HHV of 25.5 MJ/kg, a water-insoluble fraction of 18.7 wt %, a viscosity of 43.6 mPa s at 40 °C, a mean molecular weight of 392 g/mol, and metal concentrations lower than 0.14 wt % on a dry basis (db). Less optimal oil properties with respect to industrial applications were observed for oil samples obtained at 475 and 625 °C. Char properties of the 575 °C sample were an ash content of 81 wt % and a HHV of 6.1 MJ/kg db. A total of 95% of the sewage sludge phosphorus content was recovered in the char. The solid waste amount (char compared to sludge) was reduced to 52% on a bulk volume basis at the pyrolysis temperature of 575 °C.

Distribution behavior and risk assessment of metals in bio-oils produced by liquefaction/pyrolysis of sewage sludge

“…Heavy metals distribution studies proved that heavy metals in SS would be mostly concentrated in the bio-char after pyrolysis/ liquefaction (Kistler et al 1987;Yuan et al 2011a;Fonts et al 2012;Trinh et al 2013). Most heavy metals like Pb, Cr, Ni, Cu, Zn, and Cd in SS were retained in liquefaction residues (bio-char) (Yuan et al 2011a;Leng et al 2014).…”

“…Most heavy metals like Pb, Cr, Ni, Cu, Zn, and Cd in SS were retained in liquefaction residues (bio-char) (Yuan et al 2011a;Leng et al 2014). Likewise, heavy metals with high boiling points (Kistler et al 1987;Trinh et al 2013) and alkaline metals (Chiang et al 2014) were mostly retained in the bio-char after pyrolysis. However, heavy metals with low boiling points like As, Hg, Cd, and Se significantly released during pyrolysis at temperature above 550°C, while other kinds of metals released with varying lower extents (Kistler et al 1987;Trinh et al 2013).…”

“…Likewise, heavy metals with high boiling points (Kistler et al 1987;Trinh et al 2013) and alkaline metals (Chiang et al 2014) were mostly retained in the bio-char after pyrolysis. However, heavy metals with low boiling points like As, Hg, Cd, and Se significantly released during pyrolysis at temperature above 550°C, while other kinds of metals released with varying lower extents (Kistler et al 1987;Trinh et al 2013). The released part of the metals would distribute in the bio-oil (Chiang et al 2014;Leng et al 2014) and possibly, the gas product (gas product only possible for high-temperature pyrolysis).…”

“…The factors determining the quantities and qualities of the products include reaction temperature, heating rate, composition of feedstock, solvent (solvent only for liquefaction), etc. (Park et al 2010;Trinh et al 2013). …”

Speciation and environmental risk assessment of heavy metal in bio-oil from liquefaction/pyrolysis of sewage sludge

“…Several studies have been focused on the behavior of HMs in SS during liquefaction or pyrolysis (Devi and Saroha, 2014;He et al, 2010;Trinh et al, 2013;Yuan et al, 2011). The pyrolysis/liquefaction process can enable HMs in SS to be concentrated mostly in one of the main products, bio-char (Trinh et al, 2013;Yuan et al, 2011).…”