Sulfur emission in the form of SO2 in flue gas is the most serious pollutant associated with coal combustion. Calcium carbonate sulfur scrubbing, the process most commonly in use today, is costly, produces a large amount of waste (dirty gypsum) and leaves a considerable amount of SO2 in the gas. The carbonate eutectic method for removing SO2 from flue gas at 450-650°C was initially proposed in the 1970’s but despite its great efficiency could not be used due to the complexity of the carbonate-sulfate melt regeneration stage. We propose a simple way to remove sulfur from the carbonate-sulfate melt by electrolysis with the produce of elemental sulfur in the cathode space of the electrolyzer at 800oC.
Thermodynamic calculations show that calculated electrochemical sulfates decomposition potential to elemental sulfur is 0.9-1.3 volts at a temperature of 800oC. Real electrochemical sulfates decomposition potential was measured in the system (Na,K,Li)2CO3 - (Na,K,Li)2SO4 by linear voltammetry method. It was found that the decomposition potential is not depend on the sulfur content in the melt (from 1.1% to 7.2%) and equal to 0.65 volts at 800°C.
Interactions in carbonate-sulphate system were analyzed by cyclic voltammetry. The drastic current density increasing with a potential decreased of less than - 0.6-0.7 V shows on the sulfur recovery from the melt. The strong increasing of current density at the same potential value was observed for various initial sulfur content in the melt.
Laboratory setup for a long electrolysis was built. For prolonged electrolysis we used the titanium electrolytic cell; gas space of the anode compartment was separated from gas space of the cathode compartment, but carbonate-sulfate salt melt was a common. The initial amount of melt was 500 gr. Both electrodes were made of titanium. Before the beginning of the electrolysis, both cathode and anode compartments (free volume above the melt per compartment 500 cm3 approximately) were purged by N2. The applied voltage to the electrodes was 0.9-1.0 volts, total current through the system was 4-5 A. The duration of electrolysis - 20-50 hours. The Ti container and the Ti electrodes did not show signs of corrosion. Experiments were carried out for system (Na,K,Li)2CO3 - (Na,K,Li)2SO4 with elemental sulfur production in the cathode space of the electrolyzer at 800oC. Initial sulfur content was equal 4.5% and 7.2%. The reduction rate was 1.9*10-6 mole sulfur/coulomb for 7.2% sulfur and was 1.6*10-6 mole sulfur/coulomb for 4.5% sulfur.
The proposed modification of the carbonate melt flue gas desulfurization method introduces two essential improvements: a) The electrochemical removal of sulfate from the melt represents a simpler, one-stage process. The reduction rate is sufficiently high that the reaction chamber can be small. b) Removal of sulfur in the form of elemental sulfur is especially advantageous since the latter is not toxic and can be readily stored.
One can anticipate that, following implementation of these improvements, the carbonate melt-based SO2 removal technique may become a practical and viable method for limiting sulfur emission to the atmosphere.