Oxidation of high sulphur coal. Part 2. Desulphurisation of organic sulphur by hydrogen peroxide in presence of metal ions

This paper describes a simple oxidative method of desulphurisation of coal organic sulphur carried out by using hydrogen peroxide as an oxidant. Desulphurisation achieved up to the extent of 3.70–6.25 wt% in 1–4 h whereas at 24 h, it increases to 9.24 wt%. When metal ions are externally added to the system containing coal and hydrogen peroxide, desulphurisation is reduced considerably in the presence of Ni2+ ion while the presence of Cu+, Co2+, Sn2+, Pd2+ and Sb3+ ions enhance the rate of desulphurisation in 1–4 h. At 24 h, except Cu+ and Sn2+ ions, the presence of other metal ions increases the magnitude of desulphurisation. Highest desulphurisation is obtained in a short time period with Pd2+ ion (28.14 wt%) whereas it is 31.12 wt% in the presence of Sb3+ ion at 24 h. The desulphurisation reaction follows pseudo-first order kinetics in all the systems and the rate constants fall in the range of (1.02–8.68)×10−5 s−1 implying a slow, steady process which is supported by the exceptionally low frequency factor. The activation energy for desulphurisation in the systems falls in the range of 8.74–13.67 kJ mol−1. The positive value of Gibbs free energy of activation represents a non-spontaneous reaction. The reaction proceeds with the absorption of heat (endothermic) as revealed by the positive value of enthalpy of activation. Sufficiently low positive values of entropy of activation and frequency factors attribute an associated reaction and proceed through the initial formation of activated complexes. Model study reveals that the process is effective with aliphatic sulphur compounds. The non-spontaneous nature of the reaction is largely due to the resistance offered by the macro-aromatic sulphur compounds. Pragmatic desulphurisation has been achieved in the process developed here and is therefore of considerable technological interest.