Producing low-ash coal by microwave and ultrasonication pretreatment followed by solvent extraction of coal

Abstract

Low ash clean coal was produced from Indian coking coal through microwave irradiation or ultrasonication followed by solvent extraction in N-methyl-2-pyrollidone (NMP) containing smaller amount of Ethylenediamine (EDA) or Monoethanolamine (MEA). Pretreatment by ultrasonication followed by extraction with only NMP gave 10–15% higher clean coal yield of (56–58%) than extraction with only NMP (without pretreatment). Effect of using up to 10% EDA as cosolvent in NMP led to maximum clean coal yield. However use of EDA concentration beyond this showed decrease in clean coal yield and increase in ash contents. Clean coal yield increases because EDA is a strong polar solvent and strong base, which disrupts coal interactions and breaks bonds in coal by acid-base interactions. The extraction yield with and without pretreatment and microwave irradiation led to the enhancement in the yield of clean coal and was found to be in the range of 66–70% with microwave irradiation, which was 3–4% higher than without pretreatment. Clean coal Free Swelling Index (FSI) was improved by one unit. This was demonstrated at lab scale. Subsequently, this process was scaled up to 40 kg/batch coal mini-pilot plant.