An improved tar–water separation process of low–rank coal conversion wastewater for increasing the tar yield and reducing the oil content in wastewater

Abstract

Complete removal of tar from wastewater is one of the most challenging problems in the pyrolysis of low–rank coal. In this study, the properties and composition of tar are comprehensively analyzed, and the reasons for the difficulty of oil–water separation are expounded. A separation method using tar fraction as extractant is also developed. Results show that the oil content in the wastewater can be reduced to less than 300 mg·L−1 when the boiling point of the fraction is 120–140 °C and the volume ratio of the extractant to the wastewater system is 1:4 at room temperature without adjusting the pH value of the wastewater. This condition can meet the requirements of follow-up treatment devices and reduce the biological toxicity of wastewater. The revamping scheme and important operation parameters of the distillation unit are optimized through simulation. The proposed method is simple, has low investment cost, and does not require buying chemicals. The method exhibits good industrial feasibility and is expected to solve the problem of oil–water separation in coal pyrolysis.