Hydrothermal Carbonization of Crude Oil Tank Bottom Sludge for Energy Applications

Abstract

Hydrothermal carbonization (HTC) is a promising technology for the conversion of waste biomass into an energy-dense coal-like product known as hydrochar. This study evaluated the application of HTC to generate a renewable solid fuel resource from hazardous oily sludge. The influence of process parameters on the energy characteristics of hydrochar was investigated. A design matrix of 20 experimental HTC runs was used as input for response surface methodology (RSM). The central point for this HTC operation was at a temperature of 200°C, with a duration of 75 min and a concentration of 5.5%. The higher heating value (HHV) increased from 18.18 to 28.7 MJ/kg. A linear correlation was derived between yield and HHV. The influence of process parameters on energy densification and energetic retention properties revealed that energy storage properties were enhanced. In total, 66.6%–80% of energy was retained in hydrochar. The enhancement of the fuel characteristics of hydrochar was demonstrated by the elevated fuel ratio of 0.31 obtained at a temperature of 250°C, thereby indicating the improvement brought about by the HTC process. The quantification of energy losses due to convection and radiation yielded 309 and 1,854 kJ/h, respectively. Maximizing the efficiency of internal heat sources is crucial to conserving energy and reducing costs in HTC. The activities of the oil industry have several impacts on the environment due to the large amounts of oily waste generated. Oily sludge is a prevalent hazardous waste generated in the petroleum industry, and effectively treating it remains a key challenge for the petroleum and petrochemical sectors. Hydrothermal carbonization is an important thermochemical conversion process that can be used as an energy-efficient alternative to enhance the dewaterability of sludge and meanwhile to convert sludge into high-value-added products, such as clean biofuel, organic fertilizers, and precursors of functional materials. Optimizing the process conditions is essential to successfully implement the HTC processing of oily sludge in real field conditions. In this study, waste oily sludge was converted to an energy-rich product, namely, hydrochar. For this, the study investigated the hydrochar characteristics for hydrothermal conditions including a reaction temperature of 175°C–250°C, a reaction time of 30–120 min, and a solid load of 3%–8%. The study also found that the majority of energy loss occurred during the cooling process and therefore suggests that this process might be operated as a continuous process rather than a batch process.