Degradative solvent extraction of low-rank coals by the mixture of low molecular weight extract and solvent as recycled solvent

Abstract The authors have proposed a degradative solvent extraction method to dewater, upgrade and separate low-rank coals to upgraded coal, high molecular weight extract (Deposit), low molecular weight extract (Soluble), and a little amount of liquid and gas products. For the practical application of this extraction method, the solvent must be recycled. Furthermore, the separation of the Soluble and solvent, which was implemented by distillation, consumed much energy and added the cost of the whole system significantly. In order to solve these issues, the utilization of the Soluble and solvent as the recycled mixture solvent was proposed and investigated in this work. The results showed that the yields of the Deposit increased by more than two times with the mixture solvent recycle, and got almost stable after 5–7 extraction cycles. This made up for the loss of the part of the extraction product (Soluble) to a large extent. Furthermore, the ratios of O/C, N/C, and S/C of the Deposit decreased with the mixture solvent recycle, indicating that the Deposit was further upgraded. The aromaticity and reactivity of the Deposit decreased and increased with the mixture solvent recycle, respectively. The C al /C ar of the Deposits ranged from 0.35 to 0.84. The ash content of the Deposit increased slightly from 0.25% to 0.29%, and then got almost constant after 4 times of the mixture solvent recycle. Only 0.2–0.4% of the main inorganic elements were transformed from raw coal to the Deposit. The main inorganic elements in the Deposit were Si, Al and Fe. The relatively high contents of some inorganic elements in the Deposit were caused by their high contents and existing form in the raw coal. Thus, it was showed that using the Soluble and solvent as recycled mixture solvent is a feasible way for the practical application of the degradative solvent extraction of low-rank coals.