Pyrolysis/combustion potential and heavy metal risk of oily sludge and derived products in industrial scale

Abstract

Pyrolysis of oily sludge (OS) has been widely applied for harmless disposal in industrial scale with the benefits of high removal and recovery efficiency of total petroleum hydrocarbons (TPHs). However, seldom research focuses on the evaluation of solid products from industrial apparatus. This paper devoted to evaluate the TPHs removal effect and optimizing directions of this technique by assessing the pyrolysis/combustion potential and environmental risk of OS and derived products, including fly ash (FA) and pyrolysis residue (PR). Unexpectedly, FA and PR still carried trace TPHs, ca. 0.27 and 0.24 wt%, which should arise attention. In comparison, the maximum weight losses of combustion for these solid samples were all higher than that of pyrolysis at the same final temperature, ca. 600 °C. Oxygen-rich atmosphere promoted decomposition of deposited coke and degradation of TPHs, especially heavy fractions. Therefore, these products had a certain potential for combustion utilization. Although the total weight losses of FA and PR were similar with that of OS, the comprehensive combustion characteristic index and combustion stability index of OS were much higher than those of FA and PR, indicating TPHs were more inclined to be burned that coke and dissociated metal compounds. Higher heating rate caused an increasing trend of these indexes. The thermal decomposition kinetics study based on the most probable mechanism function showed that FA and PR followed the mechanism of random nucleation and subsequent nucleation growth. The wastewater produced during gas washing was measured that contained large amounts of organics and harmful elements, which should be properly treated before emitted. Heavy metals are the dominant sources of environmental risks of solid products. Pb and Cu in FA and PR showed higher leaching concentrations and posed severer environmental risk than those in OS, which originated from accumulation effect. However, Cr, Ni, and Zn became more stable and safer after thermal pyrolysis/desorption treatment. These information on solid products and wastewater from industrial OS pyrolysis reminds future work to concern on the final disposal of the output of the whole technique.