Kinetic and thermodynamic studies on oxidative desulphurisation of organic sulphur from Indian coal at 50–150°C

This paper reports a relatively simple low-temperature non-isothermal oxidative desulphurisation of coal organic sulphur by weakening the CS bond using HgCl2 solution to an inorganic sulphur-free high-sulphur Indian coal. When oxidised from 50°C to 150°C in air under normal atmospheric pressure, there is continuous decrease of organic sulphur content in the samples of the feed and Hg-treated coals. Desulphurisation is more in the Hg-treated coal (4.97–14.53 wt.%) than in the feed coal (3.72–10.93 wt.%). Kinetic study reveals that the oxidative desulphurisation process follows pseudo-first order kinetics and the rate constants have been found to be in the range (3.09–5.06)×10−5 s−1 for feed coal and (4.19–6.80)×10−5 s−1 for Hg-treated coal. The activation energies for the sulphur loss reaction in the oxidative desulphurisation process by using the pseudo-first order kinetic (feed coal: 2.21×102 J mol−1; Hg-treated coal: 1.53×102 J mol−1) have been found to be almost similar to those calculated by applying the Coats and Redfernʹs equation (feed coal: 2.19×102 J mol−1; Hg-treated coal: 1.53×102 J mol−1). However, the value is higher (feed coal: 3.50×102 J mol−1; Hg-treated coal: 2.70×102 J mol−1) when Horowitz and Metzgerʹs equation is applied. The frequency factors computed by the pseudo-first order kinetics are very low and have been found to be 2.66×10−5 s−1 for feed coal and 3.96×10−5 s−1 for Hg-treated coal, suggesting very low rate of successful collisions for the formation of the activated complex. Evaluation of thermodynamic parameters viz., ΔH, ΔU, ΔS and ΔG, reveals that this oxidative desulphurisation process is non-spontaneous in nature and the degree of non-spontaneity of such a process in the feed coal is more relative to that of the Hg-treated coal.