Electron transfer process. Part 2. Desulphurization of organic sulphur from feed and mercury-treated coals oxidized in air at 50, 100 and 150°C

Abstract

In this paper, experiments have been undertaken to understand the nature of organic sulphur compounds retained after oxidation at 50,100 and 150°C for both feed and Hg-treated coals as well as the extent of desulphurization by the electron transfer process. It has been observed that oxidation of coal in air resulted in the predominant formation of disulphide compounds. Besides these compounds, aliphatic thiol compounds seem to have been leached out by the electron transfer process. Maximum desulphurization achieved by the electron transfer process carried out in absence of naphthalene is 19.17wt% in feed coal and 17.78wt% in Hg-treated coal of the total organic sulphur for the samples oxidized at 100°C. The desulphurization reaction has been found to follow pseudo-first-order kinetics and their rate constants have been calculated. The activation energies and frequency factors calculated for these coal samples are found to be in the range 21.28–25.53kJmol−1 and 5.01×10−4–28.18×10−4min−1, respectively. Kinetic studies reveal that the desulphurization reaction is non-spontaneous in nature which is further supported by the negative value of ΔS and positive value of ΔG. Low frequency factors and negative values of ΔS suggest that desulphurization by electron transfer process is an associated reaction. Although the positive value of ΔH attributes the endothermicity of the desulphurization reaction which requires input of energy for effective desulphurization, but the energy liberated during the formation of ion-radicals is utilized for rupture of bonds in those organic sulphur compounds which contain relatively weak C–S as well as S–S bonds. Total desulphurization (summation of oxidative desulphurization and desulphurization by electron transfer process, in absence of naphthalene) of organic sulphur is found to be 27.38wt% in feed coal and 28.45wt% in Hg-treated coals. This suggests that the presence of Hg2+ ion and naphthalene has no significant influence on desulphurization. The results of this work have considerable technological interest.