Desulfurization of oxidized Indian coals with solvent extraction and alkali treatment

The north eastern region (NER) of India has a substantial deposit of high sulfur (2-8%) coals with low ash and high volatile matter contents. Assam, one of the coal producing states of the region, has coal deposits with 75-90% of organic sulfur functionalities affixed to the coal structure and difficult to remove. The present investigation reports desulfurization of oxidized Assam coals by solvent extraction and alkali treatment separately. The two coal samples are selected to represent two categories: one with high organic sulfur and low ash (Baragolai) and the other with low organic sulfur and high ash (Ledo) coals of the region The desulfurization routes are comprised of oxidation of two coal samples of various size fractions in an oxidative medium (H2O2 and HCOOH) followed by solvent extraction (dimethyl formamide, DMF) and alkali (NaOH) treatment separately. Oxidation of Baragolai and Ledo coals removes up to about 84% of inorganic and 28 and 18.5% of organic sulfur, respectively. After DMF extraction, desulfurization increases up to 95 and 93% for inorganic sulfur and 31 and 23% organic sulfur of the oxidized Baragolai and Ledo coals, respectively, while the alkali treatment shows complete removal of inorganic and a maximum of 33 and 26.4% organic sulfur for these coals, respectively. During oxidation, solvent extraction, and alkali treatments, ash removal for both the coals is observed. Desulfurization is found to increase with decrease of the particle size. Decrease in the atomic S/C ratio of oxidized samples shows the effectiveness of the process. Fourier transform infrared (FTIR) spectroscopy studies show formation of sulfones and sulfoxides (SO) during oxidation and their removal by solvent extraction and alkali treatment. With activation complex theory (ACT), the Gibbs free energy of activation (Delta G*), entropy of activation (Delta S*), enthalpy of activation (Delta H*), and activation energy (E-a) calculated for organic sulfur removal show possible removal of low molecular weight sulfur compounds. The nonspontaneous nature of the reaction is mainly due to the steric hindrance by large aromatic rings.