Reduction of thermal coals environmental impact by nitric precombustion desulfurization

Sulfur content in coal is converted into sulfur oxides during combustion which, among other effects, causes acid rain. To reduce this environmental impact, precombustion coal desulfurization by means of nitric leaching has been researched. Two Spanish high-rank thermal coals have been treated at laboratory scale using nitric acid concentrations of 20 and 30 wt %, at 90 degrees C, and with residence times ranging from 5 min to 2 h. The tests have been carried out at atmospheric pressure. The evolution of total sulfur, pyritic, sulfate, and organic forms of sulfur and also the modification of ashes, nitrogen, and calorific value have been determined by applying European and ASTM standard methods. Results demonstrate that pyritic and sulfate sulfur are practically eliminated but organic sulfur is affected very little by the treatment Ash is reduced, mainly by dissolving cabonates and sulfides, while the nitrification produced is moderate whenever the times of treatment are controlled. The calorific value expressed per unit of organic carbon practically does not vary. The coal energy recovery has been calculated using the experimental data for mass recovery and calorific value and demonstrated that, in this precombustion desulfurization process, very small losses of the total coal energy are produced.